This question seems to come up a lot in the people starting out in virtualization. Sadly there is no quick and definite answer. The best and most accurate way of sizing the solution is having a partner / vendor come onsite and perform a Capacity Sizing Assessment. this is true of any hypervizor platform. There are a number of tools out there to assist with this. That being said here is a great link to read as not only will you want to know how many VM’s you can install but what the underlying hardware’s impact may be on that application.