One of the eternal questions in role playing is determining the proper size of a group. How many players can you have in a group? How many is too many, and conversely how many is too few? There are a couple of things to consider when coming up with an answer.
How many can the GM handle comfortably? That is the biggest issue to answer. This depends on the players of course. I have seen large groups that work because everyone is really into the game and focused. I have also seen small groups where players still had a hard time getting into the game. So it really comes down to the GM. What are they comfortable with? I personally am okay with the 4 to 5 player range. More than that and I begin to fret about getting everyone involved. Fewer than that and I have trouble gauging the combats. How dangerous should I make the combat?
What kind of game are you running? Is it a combat heavy, hack and slash, dungeon crawl kind of game? In that kind of game, the more players involved does not necessarily create problems so long as you manage the combat rounds efficiently. On the other hand, if you are emphasizing investigation and storytelling, a large group can really slow things down. It is much easier to run that type of game when you only have a few players.
How reliable is your group? Say you only have 2 to 4 players, and one gets sick, the game almost has to be put off. So the less reliable the group the more players you want. So if one person cannot make it there are still enough players to play.
All of these things also affect how you plan and run your game. With a small group, you almost have to design your game around the possibility that it could be interrupted. Or have a few options for side games when one person is gone. If it is a larger group the loss of one player is not as noticeable. Another point to consider in this is if you can play without a certain character. When you design the campaign and a set of encounters, can the group handle or even proceed without one character?
I personally try to plan my games so that they can handle the loss of one player. With the caveat being that if Jimmy is not well I will not run the game as I run the game for him. If necessary I will take control of the missing character. But I tend to run more encounter heavy games than narrative ones, so it is easier to absorb that loss.
Like I said, it is not an easy answer. There are a lot of things to think of when determining how may people to invite to your game. Just another thought to ponder on.