I have different standards for disc based games and cartridge based games.
I won't buy a disc based game unless its complete in box with manual. For cartridge based games, I don't really mind whether the game is complete or not. If there's a cart in good condition that costs less than a CIB alternative, then I'll buy the cheaper game. If there's a fiver in the difference, then I'll probably bite the bullet, and get the complete game.
I don't buy games that I don't intend on playing or have some interest in playing. I don't understand people trying to own a complete NES collection, but that's not for me to understand. If it makes them happy, then who am I to tell them otherwise?
EDIT: Also thought it worth mentioning that I don't collect PC games. If I buy a PC game, it's usually just a digital download. The only PC games I actually buy are Valve games... LOL.
I collect for the NES, SNES, N64, Gamecube, Wii, GB, GBC, GBA, DS, 3DS, PS1, PS2, PS3, PSP, Xbox, Xbox 360, and Mega Drive.
I have 3 games for the Wii U, but I bought the console for my Mum, so I don't really collect for it, she does. I'll probably pick up the games I want to play, but it will never be a "main focus". I'll probably start collecting for the PS4, but I'll most likely skip the Xbox One, not because of the shit they stirred with their old used game or DRM policy, but I just want to focus on one console this generation. Collecting for all three last generation was a bit of a blur.
To answer OP's original question. I'm a player first, a collector second. I'll never be one of those guys with over 100 games for one system. I'll most likely keep it between 10-50 games per system. I only want like 7 games for the GBC! LOL.