Depends on the game, but usually one or two. If it's a free-standing game, you could either put out one big book that gives the general rules, flavor, and setting, letting the DM fill in any smaller scale details, (White Wolf has been quite successful doing just that,) or else two books, one of classes, basic powers, common rules, general setting, etc. for the players, while the DM's book would go more in-depth with maybe a default play location, monsters, gear-making tips, and the like that the players wouldn't have much reason to look up often. (Keep in mind that the Deep Secrets of the campaign world should not go into that book; players will buy and read it, it's just stuff that doesn't apply to the characters and their sheets directly.) Of course, suppliments may well Rifts the game out, but so long as the company keeps most of its metaplot from being obvious to the newbies on the surface of the world, saving massive changes for revised versions of the core book, things should go fine.
If it's a D20 product, one. Or four, if you count the PHB, DMG, and MM. The basic book should be there to whet the appetite, giving general information on the area, generic setting-specific monsters, spells, feats, items, prestige classes, and so on, all in one big book. If people want more information, suppliments should be the way to do it, but again, no game should make buying suppliments a requirement to either keep up with the power curve or the metaplot. I know it's my game world and all, but it's still disheartening to find out that the Smurfworld book I didn't think I'd have any need for had Venger oust Gargamel as the main setting baddie.