to substitute 2c of butter for oil how much oil do i need??

If the butter is in a baking recipe ie cake, then butter provides the means whereby the cake batter is aerated. Aeration plus the leavener in the cake makes it rise. It also is influences the structure or crumb of the cake.

Substituting oil for butter means considerably less rising and a closer denser crumb structure.

That being said: 2 cups butter is 80% fat content which equals 1.6 cups oil. You will have to adjust the moisture content because you are omitting 0.4 cups liquid.