250 users is not large for Squid. Any of the production releases should be able to handle that many without causing much of a CPU bump on modern hardware. I think you can start with one Squid process and expand to more if you find it stressing the machines. More likely you will need more DansGuardian proxy processes though, that is where the heavy CPU consumption will occur.
BTW, have you considered other content filtering solutions which use ICAP to integrate with Squid? It could possible be a less burden for you hardware as ICAP (and eCAP specifically) has ability to early opt-out of traffic that does not need to be filtered.
BTW not all content filtering solutions are implemented as processes some (like qlproxy) do thread based connection handling thus (possibly) decreasing CPU and (especially RAM) requirements...
Best regards, sich
