How many Jails (virtual PC’s) can you install under FreeBSD?

At least 1000:  The night of 1000 jails

The cheap (both in $$ and resource requirements) OS-level virtualization technology present in FreeBSD for nearly 10 years now. Behold… the bootup of 1,000, count them – 1,000 virtual machines on a single host with 4 GB of RAM