There is no practical limit built into Windows NT 4.0.
The number that your Print Server can support depends upon:
The number and speed of the processor(s).
The speed/throughout of the servers disk sub-system.
The speed of the NIC(s).
The speed of the printers.
The printer driver software efficency.
The size of the print jobs.
You might want to see Optimization and Tuning of Windows NT 4.0.