Can I suggest you actually perform those searches I suggested and look at that page. The hardware that happily supports 100 users in one network may not be enough to support 10 users in a different network.
Here's a bunch of things mentioned elsewhere that affect the answer:
Type of NICs - server grade NICs have a lower performance impact than desktop NICs. Some cards are known to be a bad choice.
Traffic profile - of that 100 Mb/s how much do you actually use? How much would you be pushing through pfSense? What does that traffic look like, is it all long sessions, short sessions, UDP, TCP, little packets, big packets, what?
What kind of Internet bandwidth do you have, 512 Kb/s, 50 Mb/s?
What services are you running on the pfSense host?
And so on, and so on, and so on. As I said, this has been asked and answered many times before. I'm not going to repeat the answers for everybody who can't be bothered to search.