Tile trowels come in many shapes and sizes, generally starting out from a 3/16″ V-notch up to the 3/4″ U-notch. The other main consideration is where the tiles will be set, on the floor or on the walls. When setting tiles on the floor, the mortar bed needs to be a bit thicker to withstand the punishment of foot traffic.
Another consideration is how flat your surface is. If you’re laying tile on a floor that slopes a bit, you would likely back-butter or spread out your mud-bed slightly thicker than the general guide that we’ve posted below.