It depends on the size of tile and the type of tile.
Mozaics you will get away using a 3mm trowel.
6mm trowel at 12mm centres gives 70% coverage with approx 150x150 mm tiles.
8mm trowel gives 100% coverage with approx 150x150 mm tiles.
A thin bed solid bed trowel (TOPPS sell it as a shower trowel) gives 100% coverage with up to 200x150 mm tiles.
A thick bed solid bed will give 100% coverage is for tiles approx 250mm squate.
You also need to consider if the back of the till has ridges as this will stop 100 % covearge and you may need to back butter.
You could try fixing a tile to some perspex or glass and examine the coverage from under the tile.
Karl Dahm also sell some new trowels called the midi and maxi.
The midi gives more coverage than the thin bed solid bed trowel.
The maxi gives more coverage than the thick bed solid bed trowel.