How many bags of readymix will 1 bag of portland?

5 or 6.

An 80 lb. bag of ready mix concrete will make about 1/2 cubic foot of concrete.

A 94 lb. bag of portland cement mixed with 1 1/2 cu. ft of stone (coarse gravel) and 1 cu. ft. of sand will make 3 cubic feet of concrete or 6 times as much. (this is a common 1:2:3 mix)

Quikcrete ready mix is often made with 1 part cement, and 2 1/2 parts each sand and stone.

A 1:1 1/2: 3 mix is a stronger mix but you get less volume, at 5 1/2 cubic feet per bag of cement.