The number of bosses per number of levels varies in every video game, and I was wondering how many bosses people like to see in the games they play.

I would have to say that I like option #2, because boss fights are one of my favorite parts of video games and I like to see as many as possible in a game; just as long as the game isn't smothering you with them like in Alien Soldier.

Only several a world, like in SMB3 and NSMB imo.

#2, as long as they're unique.

Mixed feelings between 1 and 2.

I chose 4 but it depends on the game itself.

Only several a world, like in SMB3 and NSMB.

One only every once in a while, like in the Zelda games.

At least one boss per world like NSMB.

5. Only several a world, like in SMB3 and NSMB.

Yes. This allows for great boss design along with as many to keep you interested and not so much as to make them not memorable.

Like 1 boss every world (there's a difference between world and levels), add a mini boss here and there. What you could also do is Mega Man Style, where each world (level, technically) has one boss with the occasional mini-boss here and there, and at like the last 3-4 levels, each one has a boss in it.