The FreeBSD kernel will only allow a certain number of processes to exist at one time. The number is based on the MAXUSERS option in the kernel configuration. MAXUSERS also affects various other in-kernel limits, such as network buffers (see this earlier question). If your machine is heavily loaded, you probably want to increase MAXUSERS. This will increase these other system limits in addition to the maximum number of processes.
After FreeBSD 4.4, MAXUSERS became a tunable value that could be set with kern.maxusers in /boot/loader.conf. In earlier versions of FreeBSD, you need to adjust MAXUSERS in your kernel configuration.
If your machine is lightly loaded, and you are simply running a very large number of processes, you can adjust this with the kern.maxproc sysctl. If these processes are being run by a single user, you will also need to adjust kern.maxprocperuid to be one less than your new kern.maxproc value. (It must be at least one less because one system program, init(8), must always be running.)
To make a sysctl permanent across reboots, set this in /etc/sysctl.conf in recent versions of FreeBSD, or /etc/rc.local in older versions.