[XltSystem]

I have caught all the 145 available, but not by legitimate means.

I don't play It, so I'm somewhat naive to the entire structure of the game.

Is there only 145 to catch? If so, what happens when you do catch them all?

Yeah there were meant to be 151, but 6 aren't released, due to all the botters and snipers(spoofing location to the exact location of a rare spawn).
Once you catch all of the ones available you get this :

Spoiler:

Nothing. You can brag about it to people but i don't really enjoy doing that.