OK, for 50 users a 60B might be a little too weak.
Right-sizing is part of the magic a Fortinet partner can do for you. It depends on a lot of parameters. And nobody wants to oversize, i.e. overspend.
For an office with a couple of VPNs, some AV, 50 users, some Mbit/s WAN link I' d guess an 80C would be enough, maybe a 110C. But this really is crystal ball science.
"Kernel panic: Aiee, killing interrupt handler!"