Im feel certain it should say on the bag or instructions that came with it - 15 cuft. So if your using the 1.5cuft bags, you need 10. if using the 3cuft bags, you need 5.
Mix it all up with your castings, inoculate that puppy and let her sit for at least a month before using.
Congrats on your choice in the TGA soils. I love the hell out of them, and will be using the same pack on my outdoor this year. I think quite a few nerds are.