number of troops underwater with water mages

It is 1.

The first path in water gives you water breathing for your pretender, and an additional 1 troop. Then scales up from there.

One interesting note is that the water path water breathing doesn't seem to follow any standard water breathing breakdown (3 Human sized, 1.5 Cavalry, 1 Giant). It just takes 1 no matter what type it is. So you can drag down 1 of whatever you want as a troop type.