I neeed help with spawning only 1 mob.

I'm making a horror mod and so far its going really good. but I have an issue. you see, I made it where my mob spawns within a certain radius of me but now it spawns tons of them at a time. and I need help making it where it only spawns 1 at a time. If you try to help pls try to attach a picture or explain a fix in a very in depth guide. bc iv'e looked at other solutions and  theres always been a fix i just dont understand what it all means.

Thank you for your help