Ahhhh... I had been wondering about this too
Our number 4 is only 4 mths old... in the first couple of weeks after, I felt like 'That's it! Definitley no more!' but now I am starting to get little pangs already, at the thought of no more babies.
FWIW we were originally going to stop at 2, then 3...the plan is now for DH to get the snip but I am already wondering if we couldn't make room for another... then reality kicks in and I think, well you have to draw the line somewhere.
It doens't help that AR is a fantastic baby who is so happy & easy. I have to remind myself another just might not be!

So nope, I have no idea how you are supposed to tell when enough is enough, but I am almost surely going to be a very zealous grandma