Are you looking at the Sizing guide for MX's ?
I think of that number more as a guidance , not a hard cap on number of devices.
(I think there was an old Cisco PIX that once had a licence for number of users ... but never mind that 🙂 )
I have had a small netparty with around 100+ clients running behind a MX64 without any issues.
What I usually look at , more then recommended users, are the bandwidth numbers (and sometimes VPN scaling numbers).
From my experience, the "Max throughput with all security features enabled" number seems pretty solid, and a good marker of where the box actually performs in real world scenarios.