Probably has a lot less than in the late 1920s early 1930s - before the dustbowl. Oklahoma, TX, KS lost a lot of population during that time and people migrated to California etc.
Very pricey name. An uncommon bank but the notes go for far more due to the unusual name... I admit, I want me one.
Impressive. Nationals are notoriously difficult to valuate. Your sheet actually doubled the known population. According to T&P, there are (were) only 6 $20 Type 2 notes known for this bank. All of the 6 known notes are from a sheet of the first 6 notes (serials 000001 - 000006). http://currency.ha.com/c/item.zx?saleNo=3513&lotNo=16010 That sheet has since been cut up. But that is good news for you. Chances are your sheet is now the only one left.