BN is a little narrower then a B but idk how much Narrower

 

The Farmall BN was 8 inches narrower than the B.

From production numbers; 220,000 B's were built. BN production was 14,967.

Both had the offset seating position known as Cultivision.

Both were basically cultivator tractors; I can only speculate, that the 8 inches corresponded to crop row spacing.

 

How do u know how many were produced? Because As, Bs, and BNs were all ran together in serial numbers

 

The Farmall A and B were nearly identical tractors. The A featured an offset engine and wide front end. The offset engine was known as "Cultivision". The B had a centered engine, seat offset to the right, and narrow front end. A high-crop AV version was available.

You'll notice the numbers were used interchangeablly by both lines per year...
A
1939: 501
1940: 6744
1941: 41500
1942: 80739
1944: 96390
1945: 113218
1946: 146700
1947: 182964 (Chicago)
End: 198298 (Chicago)
1947: 200001 (Louisville)
End: 220829 (Louisville)
http://www.tractordata.com/farm-tractors/000/2/8/285-farmall-a.html

B
1939: 501
1940: 6744
1941: 41500
1942: 80739
1944: 96390
1945: 113218
1946: 146700
1947: 182964 (Chicago)
1947: 200001 (Louisville)
http://www.tractordata.com/farm-tractors/000/2/8/286-farmall-b.html