For a role-playing game, they're pretty damn important, aren't they? Some games are designed to be run with no GM - I assume the players form the tabletop equivalent of an anarcho-syndicalist commune - but players are always a must.
With D&D, the more the merrier. Particularly in "old-school" (i.e., before Wizards of the Coast) iterations of the game, having more party members means having more muscle, more bodies to soak up damage, and most importantly (from a gamer's standpoint, at least) a good buffer in case one or two players isn't able to make it. I often end up with only two or three players; in the latter case, it's usually okay, but in the former case I have to concede to reality and break out a board game or something. (Of course, there was that one time where only one player showed up...)
Even a single player might be okay, depending on the system being run. Ghost Dog: The Way of the Samurai is a perfect example; the authors acknowledge in their introduction that adults are usually busy, so the game is set up for one GM and one player. My issue is that I have zero ideas for modern-day crime plotlines. (I also take exception to the idea that character death should never happen unless it's important to the story.) Hell, even for The World of Darkness, the most I can come up with is a one-shot.
My apologies if the last few posts (and possibly the next few as well) are a little disjointed and/or short. Writing two term papers in two days, and a third coming up, have left my brain a little bit tired. I will still be doing my best to stick to my commitment for the month.