What quantity of coconut oil should be substituted for butter in a brownie mix?
Butter is about 80% milkfat, and 20% water, more or less.

3/4 stick is 6 tablespoons.

You should be able to substitute six tablespoons of coconut oil one to one.  It should be well within the tolerance of the recipe.

If you really want to account for the water, you would do about 5 tbl of coconut oil, and 1 tbl of liquid.