GTA San Andreas supports more than 32 players because the MP code was implemented from scratch for both SAMP and MTA, and the only available language is Lua. Also GTA: SA does not requires you to have a legal copy of the game.
FiveM on the other side uses the RAGE side of things and it has support for Lua, C# and JS. It has anti piracy and anti cheat by default to make sure that everyone plays fair.
Also, I have seen only one or two servers that have reached the max of players in FiveM. I don’t know why you need support for more than 200 players.