Thanks a lot for this interesting observation and test. There should be no limitation in NCS and I know users that use more than 6 cameras like this. However, strange things happens.
Since you use localhost, 127 and 192 the “external” web server (IIS ?) for the single webpage must be on the same computer as NCS. Since it is so distinct 1-6, 7-12 and 13+ can it be a limitation in that web server? I assume that you do not use the admin account for the MJPEG links.
Another interesting test to sort things out can be the following. Create several user accounts. User1 have access to camera 1-6, user2 have access to camera 7-12 etc. On the web page use the URLs with the same IP for all accounts (localhost, 127 or 192) but use the account for different users which will give different Tokens. I hope I am clear.
If you connect to NCS with the webb client and use Multi View for all cameras. Any problems?