2-3 of those bags should do it for 15 plants. I just filled 5 buckets with 6.5 gals of soiless, and 6 5 gal buckets with 6 gal of soiless. It's all still dry, so it will compact down to less, but that is also amended with 11 gal of Vermiblend soil amendment, and 5.5 gallons of hygromite, because I had a bunch of hygromite laying around, lol, and I like the added benefit potassium silicate, but it raises your pH as well, so be careful if you interested in spending a bitch-load of money on hygromite. But all together I made up 68.5 gal which will fill 10 7 gal pots nicely with.... 6.85 gal of soil each, dur. lol.
At least I think that is what it all turned out to be. These are, once again, dry measurements. But you should get roughly 50 gal of soil out of each bail of Promix. I use sunshine mix and get all my goodies from the Vermiblend, which is only 11 bucks at my favorite hydro store, but 18-28 bucs online for the same 1 cu.ft bag, shit. But the bag amends 2 whole bales of Promix/Sunshine Mix #4. I'm not sure how I feel about it yet. Some of my plants love it, the weaker ones do not like it. Long story on the weaklings.
Anyways, I probably wouldn't play around with soil amendments yet, unless you need to stretch just a few more gallons of soil to save a buck, but be careful with it, although your larger plants should be fine. But be careful not to feed the plants anything they are already getting from the amendment.
I don't remember if you said if they are all fem? Because you don't want to spend a lot of money on males.
On the plus side you can re use soiless mix if treated, which is pretty easy to do. Some people don't even really treat it, they just let it dry out and sift out the roots, and rinse it when they presoak the pots.