You don't need too much.
If you brush it on - 2 or 3 decent coats should be fine - let it dry in-between coats.
If you are unsure - when applying the mask, also apply it to areas of the shell that are redundant / that you would cut away once painted. Then you can do a test cut on the mask when it's dried and you test if you are good to go.
You want it to peel away easily in one piece. If it falls apart its too thin - put on another coat
Basically - too thin = nightmare to remove - too think - ok but costly!
Personally - I spray it on