How many Virtual Machines can you run on an ESX Host ?
This all depends on the amount of CPU & RAM that you think each VM will require. Importantly you can limit the resources available to each VM from a central management tool. This means that if you have a VM that has a bad application and consumes 100% CPU, or indeed crashes, then the other VM’s will not be affected and you can reboot this machine as if it were physical.
Typical installations in Manufacturing IT dictate that 3 ESX Hosts could easily handle about 30 – 40 Virtual Machines.