If you didn't want to make more mobs you could alternatively make the spawn method a procedure, so that when the player enters the biome it spawns one. This way, literally only one would spawn. Just make sure despawning is turned off so whatever the mob does isn't lost.
There should be max 1 of them
Since that mob is probably the only mob in your dimension the mob weight system makes it spawn a lot.
Check here for more info: https://mcreator.net/forum/53135/understanding-mob-spawning-complete-indepth-explanation
I have had this problem too, When I made a single biome world, there was too many custom entities that the game almost crashed.
So i need to make more mobs, so they will be more rare and make 1 or more biomes right?
Yeah, I think
If you didn't want to make more mobs you could alternatively make the spawn method a procedure, so that when the player enters the biome it spawns one. This way, literally only one would spawn. Just make sure despawning is turned off so whatever the mob does isn't lost.