Press demos for Serious Sam have been taking place at E3 this week, and according to a tweet from VG247 deputy editor Kirk McKeand the new Serious Sam has 100,000 enemies on-screen. Thats not in a level in total, but all apparently rendered on-screen at the same time. That number has been confirmed by a representative from Devolver Digital.
Source: N4G PC Serious Sam 4: Planet Badass will have 100,000 enemies on-screen at once