How keys are used explained in other answers already. Number of keys you can request would obviously depend on how successful is your hame on Steam. E.g I doubt Valve would generate 100,000 keys for a game with zero sales, but likely under 10,000 is possible.

Other than selling keys they can also be used for marketing. If you for instance have a game with multiplayer, lots of DLCs or IAP then giving away keys for base game make a lot of sense: even if only 1% of people who grab the key gonna play it they can still eventually buy other copy for a friend, etc.