Well, usually, after the first client is Open, subsequent clients don't take much more memory(As sprite, scripts and etc are already loaded.). So it'd depend much more on your processing power and and your network capabilitie. Press Ctr+Alt+Del to bring the Task Manager, go in Performance and then on Resource Monitor. You'll see Cpu, Network and Memory, just start one client and add a few more and see how much more it takes. Then you'll be able to guess what your max would be. (Altough i would recommend keeping your CPU usage below 75%, to prevent lag or disconnect from processing spikes.)
Edit: How dumb of me, I forgot to mention, for Network utilisation refer to the Kbps showed and compare it to your provider offered speed, the % that it give is Network utilisation of your Network Card capabilitie who is probably much higher than you're Internet speed.