The answer is how much can fit on a FAS3240. A vFiler is not a hypervisor and runs inside the physical controller similar to a container. Since there is no ONTAP instance running in the vFiler itself there is almost no overhead to a vFiler and all the load is handled by the physical controller vfiler0. A vFiler takes about 400k of system memory is all.
The answer is it depends...on the workload fo the 5 file servers which we don't know. Do you have IOPs and MB/sec and sizing info from the file servers? If not you can run any kind of capacity planner including the free netapp one (see your NetApp SE or VAR for the virtualization capacity planner) then that performance information can be used to size out what controller works... a 3240 should easily handle 5 servers (based on average) but you may have a much bigger workload or even a much smaller one.