:NSCONTROL SERVER=VTSERVER,,<newmaxservercount>

This takes effect dynamically and can be made a part of your system start up routines.

You can also edit the file VTSERV.NET.SYS. Line 8 is the default number of VTSERVERs. After editing this file, stream JCONFJOB.NET.SYS then cycle the network for the change to take effect.