As always, trying to compare the sum of a set of unknown quantities to an unknown metric is impossible. Domains (defined loosely as the activity generated by the operations of a domain) are all unique--no two are alike. Thus it is impossible to say N domains on this machine are fine, but N+1 is simply too many.
i absolutely agree with BobFarmer as its very hard to predict resorce usage.
generally its decided by trends and history but it does not guarrenty that server can host N domains in future. it might go N + m domains or even resources would be less for N - m domains
one has decide safe number, technical and commercial viable numbers.
e.g a VPS with 256 MB ram and 200 MHz CPU can host 10- 15 mid usage domains, 40 - 50 low usage domains. or this is too less for 1 high utilising database orented forum or shopping or community portal domain.