Can you have a total of 20 rooms, or can you have a total of 20 rooms on each floor?

total of 20

Ow man, that aren't a lot of rooms... i would've like to make a kind of maze dungeon, but that isn't going to be the case now, cause there are already 17 different rooms or something...

does a garden also counts as a room?

Yes Both kinds of Gardens count as rooms

and does a skill that goe sup or down, counts as one room or 2 rooms?

Every thing that has something in it is counted as a room.