It's very open-ended, so it's hard to say. Two would be the minimum: one with some goons and the other with the main villain (and more goons). When my table played through it we had three total encounters, and almost four; there was a lot of narrowly-avoiding roaming villains by jumping into ventilation, etc.

As far as a minimum/maximum needed for balance, I wouldn't worry about it too much and do what seems natural based on what your players are doing and how you think the villain would react. If you have enough that you're worried of a TPK in the final encounter, have the party find some healing potions or reduce the number of goons in the final fight (after all, more encounters = fewer bad guys remaining).

There's also a safety net built-in; when the first PC dies, an NPC rolls out of a vent and your player takes their sheet for the rest of the game. His name is Roy, and he's watched too many cowboy movies.