50 objects low poly is fine. You can do some optimizations like set animation rate to 0 or object invisible when its far away or behind walls if you want a lot of npc.
Also, not a lot of games has over 50 character/npc/enemies on screen. I made tests and 100 on screen isn't a problem which should be enough for 95% of game genres. Games like the Ps2 Dynasty Warriors games give an illusion of 1000s of units, but in reality there's only like 40-50 enemies on screen at once, further ones aren't visible and the Ai is so basic. Creating smart enemies that uses collision checks and loops and stuff usually is the performance killer if not designed correctly. Gdevelop can display 1000s enemies sure, but will lag once you start giving them behaviors or collision checks. 1000s of static object instances like trees in the background is nice tho, something Gdevelop is good at.