It's just a guess but you could probably do 75 containers with two 64 qt. bags with some left over. I WS 200 milk jugs last year from a 3.8 cu. ft. compressed bale of grower's mix but I blended my own combination of potting mix & soil in a bucket before filling the milk jugs. I found that adding a little potting mix & soil to the growers mix made it easier for it to absorb water. Since I was also new and didn't really know what I was doing, it's probably sheer luck & Trudi's method that succeeded more than I did.

This is a good question. I've went thru the large MG bag and used 23 combo/jugs/pans/coke bottles. But I mixed more peat and perlite into it, to give it more drainage.

Thanks for giving me some idea. It's interesting that some winter sowers mix soil for better water retention and some mix perlite for better drainage.