Is there a limit to the number of Windows VPN dial-ups?

I encountered a strange problem when using a Windows system for VPN dial-up.

In my work scenario, I need to use Windows system to connect to several customer networks at the same time through VPN (PPTP, L2TP, etc.) and maintain the connection:

1. If I do VPN dial-up (L2TP) on Windows Server 2012, there is no problem if the number of simultaneous dial-ups (and keeping connections) does not exceed 5
2. When there are already 5 VPN (L2TP) dial-up connections successfully, if I try to dial up the 6th VPN (L2TP) at this time, it will fail: error code 633
3. But if I make the 6th VPN (PPTP) dial-up at this time (not L2TP type), there will be no problem
4. On my Windows 10 system, the upper limit of simultaneous VPN (L2TP) dial-up is 2, and the third VPN (L2TP) dial-up will fail: the error code is also 633
5. No matter I use rasdial.exe to test in the command line, or I use the API RasDial function to test, this problem of the upper limit of simultaneous dialing will appear.
6. I tried adjusting the order of VPN dial-up, but the problem still persists.
7. I tried this solution from Microsoft, ([“Error 633 – the modem (or other connecting device) is already in use” error message when you try to establish a VPN connection][1]), but it didn’t work, I did not find that port 1723 is occupied

I have not tested on more different versions of Windows systems.

This question gives me the impression that the Windows operating system has a built-in upper limit on the number of VPNs of the same type that can be dialed at the same time? Moreover, this upper limit is different on different Windows operating systems.

I haven’t found any information related to this problem online, or am I looking for it in the wrong way?

If there is indeed a built-in upper limit, is there any way to increase the upper limit?

Thanks.