10.
Place the mushroom caps on a dimple foam pad and leave them to dry. Same as with the stems - they don't need to dry out completely but they need to be firm enough, so that when you attach them to the stem, the stem won't start sinking into the cap. The larger (heavier) the cap, the firmer it needs to get.
Before leaving them to dry I made an indentation on the bottom side of the cap (where the stem will go; it will hide the cap-stem joining).