If you think that just multiplying the part length by the number of parts needed is the answer you will be quite disappointed...

The number of these needed is more than just the number of parts times the part length in inches...

The  usually accounts for the vast majority of stock required.

But the amount of material lost by cutoff tooling(kerf loss), the first piece and remnant in the machine "steal" parts makeable from the material you purchase.

Parts can also be lost from production by failure to conform to requirements for dimension or during an extended campaign to get the set-up dialed- in.