There are 32 seeds in Grand Slam tournaments. Seedings are announced just prior to the draws being made.
Seedings are used to separate the top players in a draw so that they will not meet in the early rounds of a tournament. The top seed is the player deemed by current rankings as the strongest player in the field. The first seed and the second seed are placed at opposite ends of the draw, with the intention being that the best two players will not meet before the final.