Steve B wrote:

You can make about 5.5 cubic feet of concrete with one 80 or
88 lb bag of cement using a standard mix formula, which would
be enough to fill your form about 11 times.

That would be if I used cement, sand, lime, sand, and aggregate.

You won't need lime. And you don't need to use sand twice.

The aggregate is just crushed stone. For something with small details
like your paver form, I'd use 1/2" crushed stone - not the standard 3/4"
size.

How many cu. ft. in one 80# sack of premix?

I mentioned this in my previous post, but just to answer it here, and
assuming 5 lbs of water, then your 80 lb bag will give you about .58
cubic feet of concrete.