It depends on weather you want a DSB (
Deep Sand Bed) or a SSB (
shallow Sand Bed)
I think most consider a DSB 5 inches or deeper and less would be a SSB. So there is another month of reading the pros and cons of both untill you decide which one you want.
Once you decide you can use simple math to calculate hor many cubic feet (or inches) you will need. I know that bags have a weight not sure if there is a volume in them. I'm sure you can go to the mfg and find that out.
Also, if you havent already figured it out there are a ton of grades of substrate from super fine sand to very corase crushed coral.