What is the importance of pawns in the endgame? How important is it to have more pawns in the endgame phase? Does the power of the individual pawns decrease or increase as the game progresses and why?
Having more pawns than one's opponent (assuming non-pawn material is even) is extremely important in the endgame. Many games of chess boil down to endings where one side has just a single pawn more than the other, and all the action centers around the attempt to promote. In fact, one could not unreasonably define endgames, as opposed to middlegames, as those positions in which the main direct objective is to queen a pawn. (There are plenty of alternative definitions as well, none of which can be perfect.)

A one-pawn advantage is certainly not always enough to win, but the mere fact that having one extra pawn is very often a winning advantage shows just how important the pawn count is in the endgame, and in this sense I would say pawn "power" (to use your wording) does increase as the game goes on. It is a pretty reliable rule of thumb that the side with the material advantage should look to trade pieces and avoid trading pawns; conversely, the defender usually strives to trade off as many pawns as possible in order to increase the chances of a draw. Again, this is just a rule of thumb, and one of the beauties of chess is that one constantly encounters exceptions to such "rules," but this advice will generally steer you in the right direction.