When you upgrade a Dart Monkey into a Spike-O-Pult, the description is:
Converts the Dart Monkey into a Spike-O-Pult, a powerful tower that hurls a large spiked ball instead of darts. Good range, but slower attack speed.Each ball can pop 18 bloons.
So the dart monkey goes from popping 1 balloon (2,0 upgrades) to 18 (3,0 udgrades).
However, what happens when you upgrade on the other path, where it says the Dart Monkey can now pop 2 or 3 balloons? Does it add to the 18? Or is it useless as one upgrade pops 3 and the other pops 18?