This is probably done best through a trial-and-error method. You can "theoretically" run somewhere around 15, but you'll be using up all the ram there is, so I would probably run 10 and see how the performance is.
You didn't list what CPU you're using, which I believe will be the more limiting factor besides RAM. It will depend on your VPS users as well and if they'll be maxing out the resources at all times.
if you're trying to oversell and dissatisfy your customers you can run plenty ov VPS's on these specs...but if you aim for great service than 10-(12) as maximum...don't forget you need some RAM to run the machine, some burst ram if necessary etc...
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