As a rule, 1/2 cup per gallon...so you are looking at 28 or so cups. But you are going to put some rock in there I assume? That'll lessen your total gallons.
That'll give you a SG in the .021 - .023 range, if you are running a reef you may want to bump that up to 1.025.
Start w/ 1/2 cup per gallon, bring it to temperature, test, and adjust as necessary. Don't even try this hobby without some means of measuring your SG, preferably a refractometer.