How many MT4 you can run on your VPS depends on a few things.
How many symbols are you receiving data for?
How many charts are you running on each MT4?
How many indicators on each chart?
How many EAs on each MT4?
How many open trades do your EAs concurrently manage on average?
Are your EA’s or indicators cpu intensive?
It is best to run each MT4 on a VPS as “lean” as possible. Run only exactly what is needed and nothing more. For example when using a trade copier EA such as ours (at SimpleTrader.net) that means 1 chart and 1 EA in total per MT4. Do not run a single indicator or EA you don’t have to. Hiding symbols that you know for sure you will not trade is recommended. This will save memory and cpu usage.
Memory (RAM) is very important to monitor when you have a VPS. Monitor your memory by using Windows Task Manager (shortcut CTRL-SHIFT-ESC). On the Performance tab of Windows Task Manager make sure the “Available” and “Free” fields (in Windows 2008) don’t show that you’re running too low. We consider low as being less than 150Mb in aggregate remaining. If you need more memory for your VPS inquire on an upgrade with support.
Windows Task Manager (2008)
Windows Task Manager (2012)