At these early stages of prototyping, with game design decisions abound, the question of 'how many' is very prevalent. Every part of the game design begs this question. How many classes? How many abilities? How many entrances to a part of the map? How many players? How many points? How many types of X? How many woodchucks?
Ever wonder why football has 11 players on the field? Why are field goals worth 3 and most touch downs worth 7? 11 players in soccer? 3 outs in baseball? 5 pitches for a full count? 5 players in basketball? 5 colors in "Magic: The Gathering"? 3 factions in the best RTS games? 3 possibilities in "Rock-Paper-Scissors"? Chuck wood, who chucks wood?
The answer to all these questions is simple: Prime numbers. Prime numbers force conflict. Prime numbers unbalance the game and encourage players to push on the imbalance. When designing a game and you have the question of 'how many', fall back to a prime number. So for me, when I was trying to figure out how many base chassis we should have, I started with 3. When I needed to decide how many variations on a skill we needed I started with 7 and then cut it down to 3. (More on what 'chassis' and 'skill' means in our game at a later date.)
Prime numbers are a guideline, not a rule. The best game design guidelines are the one that the great designer knows when to break. Break this guideline, on occasion, and you'll be left with an even more interesting mechanic. One could say that the NFL got more interesting when they started allowing the two point conversion. All of a sudden we have a game that every once in a while an 8 or 6 point touchdown adds an interesting wrinkle to the whole game.
Level designers should take note of this idea as well. The best level designs also use prime numbers. I have something I call 'The rule of 3' and that is: Every main combat area on a level should have three different entrances per team. Again, this forces conflict. Break this 'rule' if you want to tilt the favor to one team over another. "Team Fortress: 2" does this very well. The "Call of Duty" series also has spectacular level design in this regard.
With this knowledge, you will be primed for success. Good luck.