You should be able to start as many as you want.
Mind, for best responses you need a mult-core system - if you have a quad core processor, you SHOULD be able to run two VMs (one cpu each) reasonably - as long as you also have enough memory (4G or more). If all VMs are simultaneously busy, it would help to have each virtual disk on a different drive.
But for testing things, that isn't necessary.
The reason I say two is that one CPU can be allocated to each VM, one CPU can handle VM hosting duties, and one CPU is still available for general desktop use. This isn't an exact assignment, but just a rough rule of thumb. I have a dual quad with 8G of memory, but if I run more than 4 then I find the native desktop stuff starts to lag a bit, and the bottleneck seems to be the disk (all VMs use the same physical disk for their root filesystems)
Last edited by jpollard; 02-08-2014 at 02:22 PM.