There are a lot of parts into the game where i just dont know what to do with the die. For example, when I have to roll a 3+ to cross a bridge, how many dice is it when it's not a might or speed or sanity or knowledge roll?
The number of dice you roll is equal to your stat for the roll you need. So if it tells you to make a might roll, and your might is 3, then you roll 3 dice.
There are a few separate rolls that aren't based on a trait. Usually, if there's no trait to check it'll tell you how many dice to use. This includes events such as the granny on the telephone and rooms such as the mystic elevator (both 2 dice, from memory).
One common type of roll that doesn't use a trait is the haunt roll, which you roll after each omen card is drawn. As detailed in the general rules, a haunt roll is always 6 dice, and succeeds (doesn't trigger the haunt) if the result is at least the number of omen cards drawn so far. Since you can roll up to 12 on 6 dice, there are 13 omen cards (and omen room tiles) to guarantee that a haunt will always happen eventually.