Traditionally there are 4 sections per foot on a sickle bar. Older John Deere headers will have these three inch sections. Most of the newer John Deere units use a 4 inch section and have a 4 inch stroke on the knife drive so they will only have 3 sections per foot. Most often these sections have 4 cutting edges rather than two.
End sections are often special as are some in the overlap in a double drive unit. Not really the answer you want, but it will get you close.