As a general rule reserve one core for the physical machine and the rest can go to the VM. You don't want them fighting over resources. Also make sure the VM is leaving enough memory for the physical machine to not be slowed down.
Good advice from schwinfinnegan.
A lot depends of course on what the host machine is doing otherwise. I'm afraid you're probably using for more than just being the host. Schwinfinnegan's advice makes the most sense for a host machine that runs multiple guest OSes / virtual instances.
Thankfully the host machine is mainly for FileMaker Server. There is a calendar server running on it as well but that's it. Most of the time its idle.
Machine has got 16 gigs of RAM in it.
Though the calendar server is not much overhead Parallels or VM Ware is, You need at least 1 core reserved to run the host software.