The amount of enemies on each level are as follows:
Round 1 - 4
Round 2 - 5
Round 3 - 5
Round 4 - 5
Round 5 - 6
Round 6 - 6
Round 7 - 7
Round 8 - 7
Round 9 onwards - 8
This means, that assuming you kill every enemy and don't let any escape, you will need to get every enemy, up to and including level 15 (which will total 101 enemies).
Tips: Two-Enemy combo trick: When you have two enemies chasing you, pump up one a couple of times (without killing it), quickly turn away and then back to the enemies, pump up the second enemy, kill it, and then pump up the first one again to kill it.
Settings: Make sure your settings are set to 5 lives at the start, and that the next life bonus is set to 20,000/50,000/--,---
Fast Pump trick: Instead of standing still and pumping, which is slower, you can 'fast pump'. Basically, this involves standing near the enemy, press A to start the pump, then quickly alternate button presses of
and
/
/
/
(whichever direction the enemy is compared to your character). You'll save about 0.5-1 second each time you kill an enemy this way.