@Adrinus 
var/tribble_number = 0 //Start it as 0 or Null
/mob/living/simple_animal/tribble/New()
...()
tribble_number += 1 //is equal to itself plus one

You check if tribble_number was 50 and use that to stop procreation
if(tribble_number > 50)
if(tribble_number == 50)
if(tribble_number < 50) etc...
(This is what @Giacom said just laid out in code form)