Windows 7 (extended support only): 50 vulnerabilities: 14 critical and 36 important
CVE-2021-27095 -- Windows Media Video Decoder Remote Code Execution Vulnerability
CVE-2021-28315 -- Windows Media Video Decoder Remote Code Execution Vulnerability
CVE-2021-28329 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28330 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28331 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28332 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28333 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28334 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28335 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28336 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28337 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28338 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28339 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28343 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
Windows 8.1: 5 5vulnerabilities: 14 critical and 41 important
same as Windows 7
Windows 10 version 1903 and 1909: 77 vulnerabilities: 14 critical and 62 important and 1 moderate
same as Windows 7
Windows 10 version 2004 and 20H2: 79 vulnerabilities, 14 critical and 64 important and 1 moderate
same as Windows 7
Windows Server products

Windows Server 2008 R2 (extended support only): 47 vulnerabilities: 14 critical and 33 important
CVE-2021-27095 -- Windows Media Video Decoder Remote Code Execution Vulnerability
CVE-2021-28315 -- Windows Media Video Decoder Remote Code Execution Vulnerability
CVE-2021-28329 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28330 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28331 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28332 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28333 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28334 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28335 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28336 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28337 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28338 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28339 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
CVE-2021-28343 -- Remote Procedure Call Runtime Remote Code Execution Vulnerability
Windows Server 2012 R2: 55 vulnerabilities: 14 critical and 41 important
same as Windows Sever 2008 R2
Windows Server 2016: 63 vulnerabilities: 14 critical and 49 important.
same as Windows Sever 2008 R2
Windows Server 2019: 77 vulnerabilities: 14 critical and 62 important and 1 moderate
same as Windows Sever 2008 R2
Windows Security Updates
Windows 7 SP1 and Windows Server 2008 R2

Monthly Rollup: KB5001335
Security-only:KB5001392
Updates and improvements:

Time Zone change for Volgograd, Russia and The Republic of South Sudan.
Removed support for the RemoteFX vGPU feature. Microsoft suggests to use secure vGPU alternatives. (Monthly Rollup only)
Addressed an issue that prevents users from specifying up to 255 columns when using the Jet Text installable indexed sequential access method (IISAM) format. (Monthly Rollup only)
Fixed a Windows Backup Event ID reporting issue. (Monthly Rollup only)
Fixed security issues.
Windows 8.1 and Windows Server 2012 R2

Monthly Rollup: KB5001382
Security-only: KB5001393
Updates and improvements:

Time Zone change for Volgograd, Russia and The Republic of South Sudan.
Fixed security issues.
Removed support for the RemoteFX vGPU feature. Microsoft suggests to use secure vGPU alternatives. (Monthly Rollup only)
Addressed an issue that prevents users from specifying up to 255 columns when using the Jet Text installable indexed sequential access method (IISAM) format. (Monthly Rollup only)
Windows 10 version 1909

Support Page: KB5001337
Updates and improvements:

Fixed a "potential elevation of privilege vulnerability in the way Azure Active Directory web sign-in allows arbitrary browsing". See Policy CSP Authentication for more information.
Fixed an issue in "which a principal in a trusted MIT realm fails to obtain a Kerberos service ticket from Active Directory domain controllers (DC)".
Removed support for the RemoteFX vGPU feature. Microsoft suggests to use secure vGPU alternatives. (Monthly Rollup only)
Security updates.
Windows 10 version 2004 and 20H2

Support Page: KB5001330
Updates and improvements:

same as Windows 10 version 1909
Other security updates

2021-04 Cumulative Update for Windows 10 Version 1507 (KB5001340)

2021-04 Cumulative Update for Windows 10 Version 1803 (KB5001339)

2021-04 Cumulative Update for Windows 10 Version 1809 (KB5001342)

2021-04 Security Only Quality Update for Windows Server 2008 (KB5001332)

2021-04 Security Monthly Quality Rollup for Windows Server 2008 (KB5001389)

2021-04 Security Only Quality Update for Windows Embedded 8 Standard and Windows Server 2012 (KB5001383)

2021-04 Security Monthly Quality Rollup for Windows Embedded 8 Standard and Windows Server 2012 (KB5001387)

2021-04 Cumulative Update for Windows Server 2016 and Windows 10 Version 1607 (KB5001347)

Operating System Distribution
Windows 7 (extended support only): 11 vulnerabilities: 1 critical and 10 important
Hyper-V Remote Code Execution Vulnerability - CVE-2021-28476
Windows 8.1: 12 vulnerabilities: 2 critical and 12 important
Hyper-V Remote Code Execution Vulnerability - CVE-2021-28476
OLE Automation Remote Code Execution Vulnerability -- CVE-2021-31194
Windows 10 version 1903 and 1909: 16 vulnerabilities: 2 critical and 14 important
same as Windows 8.1
Windows 10 version 2004 and 20H2: 24 vulnerabilities, 3 critical and 21 important
Hyper-V Remote Code Execution Vulnerability - CVE-2021-28476
OLE Automation Remote Code Execution Vulnerability -- CVE-2021-31194
HTTP Protocol Stack Remote Code Execution Vulnerability -- CVE-2021-31166
Windows Server products

Windows Server 2008 R2 (extended support only): 10 vulnerabilities: 1 critical and 9 important
Hyper-V Remote Code Execution Vulnerability -- CVE-2021-28476
Windows Server 2012 R2: 12 vulnerabilities: 2 critical and 12 important
Hyper-V Remote Code Execution Vulnerability - CVE-2021-28476
OLE Automation Remote Code Execution Vulnerability -- CVE-2021-31194
Windows Server 2016: 12 vulnerabilities: 2 critical and 12 important.
same as Windows Server 2021 R2
Windows Server 2019: 16 vulnerabilities: 2 critical and 14 important
same as Windows Sever 2008 R2
Windows Security Updates
Windows 7 SP1 and Windows Server 2008 R2

Monthly Rollup: KB5003233
Security-Only: KB5003228
Updates and improvements:

Fixed an issue in the Server Message Block (SMB) protocol causing frequent crashes with error 0xA on Windows Server 2008 R2 SP1
Security updates
Windows 8.1 and Windows Server 2012 R2

Monthly Rollup: KB5003209
Security-only: KB5003220
Updates and improvements:

Security updates
Windows 10 version 1909

Support Page: KB5003169
Updates and improvements:

Fixed an issue that could make scroll bar controls appear blank on the screen.
Security updates.
Windows 10 version 2004 and 20H2

Support Page: KB5003173
Updates and improvements:

Security updates
Other security updates

2021-05 Cumulative Security Update for Internet Explorer (KB5003165)