depends on what size you have.... usually to get best air flow you have a square trunk line with round pipe coming off it....
a six inch round pipe is around 100 cfm 105
a seven inch is around 150
8 230
square is 12x8 around 500
14 x 8 is 600 etc etc etc...
your furnace has a 3 ton blower or 4 or what ever
1 ton equals 400 cfm..... so 3 ton is 1200 etc.
make sure you have enough air innnnnn and enough air outttttt so you don't have heat exchanger problems early in system....