How many VMs you can comfortably run depends a lot on what your doing with them. I comfortably run 2 Windows XP VMs on my Core 2 Duo MBP with 4 GB of RAM. I can run a 3rd at times with acceptable performance. I believe there are 2 key things to keep in mind; 1) Only give the VMs as much resources as they actually need, and 2) spread the I/O out on separate disks as best you can. If you starve the host for disk I/O, CPU or RAM you'll notice it right away.
What that translates to is likely; 1) Give each VM 1 virtual CPU (unless you are doing something really CPU intensive as confirmed by Task Manager in that VM), 2) Start with giving each VM 1 maybe 2 GB of RAM and only go up if Task Manager indicates it has run out of physical RAM (you may comfortably go higher with 16 Gig in your iMac, but start low and work your way up as needed), 3) If at all possible store the VMs on as fast a drive as you can manage and not on the Mac's boot drive.
My biggest performance boost came from moving my VMs to an external RAID 0 drive connected to my MBP via eSATA (have an ExpressCard/34 eSATA adapter). My VMs are at best 1/2 the speed, probably worse when I try to run them from my internal boot disk. You have some flexibility with a mechanical & SSD drive available, but depending on the I/O needs of the VMs you may still benefit from additional drives.
With the proper disk setup and CPU / RAM settings suggested above I would think you could run 3, maybe 4 VMs pretty comfortably.
There is a tendency when first working with VMs to crank up the resources in the VMs because it is just settings not adding hardware to a physical machine. What one can't lose sight of is that the host needs sufficient resources to run itself as well as support it's guests. If you give to much to the guests, the host starves / suffers and the whole environment slows down. With virtual machines the key is to find the right balance between what the host needs and what the guests need.