How many effects are in each room?

[Uncle Jon]

There are 24 in total, which I'm guessing the point of the game is to collect all of them to beat the game? Does each door contain two?

[Ortort]

The worlds behind the doors often link into each other. In most cases, it's possible to find yourself moving from the world behind one door to the world behind another. Also, I'd personally suggest that you don't go out of your way to look up guides on how to get the effects. I think beating the game isn't exactly important compared to the experience of wandering around.