Microsoft offers Windows 10/11 Enterprise multi-session (formerly “Windows Virtual Desktop”) which allows multiple concurrent users natively, properly licensed, with no termsrv.dll hacks.
You can install the RDSH role using PowerShell with administrative privileges: Termsrv.dll Patch Windows Server 2019 REPACK
If you want, I can:
If you are looking to "repack" or manually patch the library, these are the primary community-driven methods: RDP Wrapper Library consider these legal
Before turning to unsafe REPACKs, consider these legal, supported approaches: Termsrv.dll Patch Windows Server 2019 REPACK