I'm not sure if it's a case of unbreakable habit or faulty economics but there is an assumption that every campaign should have between 4 - 6 players. If you can fit them around the table and run a game for them that doesn't dissolve into boredom and frustration for a few members, then that's the right number to have.
Unfortunately this assumptions provides a number of issues of their own. While four players can make a reasonable tactical game, six players leads to either arguments about the tactics to take or a few silent players while the others take the limelight (more often than not).
When it comes to investigative games, even four players become unwieldy when they crowd delicate witnesses and kerb stomp the usual thug challenge. After all, it's hard to intimidate a car load of people. Since these sorts of games tend to be pretty environmentally intensive with people examining objects and looking for clue trails it can also be tricky to keep four people busy.
Generally 2 - 3 players are your best bet here as one player may struggle with the challenge of finding all the clues and figuring out all of the paths (though a handy NPC could drop a few hints to guide their way).
Finally I have found that even horror games ride with the assumption that there will be four players involved. In truth I have found that the smaller the better in a dedicated horror game. While any game, even six player ones, can have their sessions of genuine horror it is harder to create a plot of creeping disquiet that isn't gobbled up by the realities of a larger group. Therefore we shouldn't be making the exceptions to the rule the reasons for having the rule in place.
In other words, while you *can* run a horror game with 4 - 6 players, the average horror game would be best off with 1 - 2 players with a third player the upper maximum. This is especially the case if the horror game involves a lot of psychological horror (to ensure each character gets enough airtime to deal with their terrors) or stealth sequences (since there are few enough hiding places for the average adult in any one location).
The irony of our obsession with 4 - 6 players is that the investigative and horror genres have a smaller player demographic and therefore it's harder to fill out those numbers. GMs who are desperate to run a straight horror game would be best off recruiting their 1 - 3 hardcore horror fans rather than begging, wheeling and dealing with action and fantasy fans to play in the game.
So please, please, please, don't feel like you have to fit into the four player mould. There's nothing wrong with a game that runs with fewer than four players. In fact, in many of the less popular genres it's actually preferable so take it as an advantage!
After all, while any fantasy GM quickly learns that it's hard to keep to the 4 - 5 player limit with so many eager wannabe players, horror and investigative GMs can guiltlessly run with far fewer players.