I don't know. In the old days when there were a lot of one-day trips in the bid package, using the waive min days between work blocks to one day, you could get one day trips all month long and if there is one is a day off between each trip using the waive, then each trip is also a block. However, in a 31 day bid month, the most blocks you would get would be 16 if you had all separated one day trips/blocks.

The contractual min days off is 12. A bid month of 31 days minus 12 is 19, so maybe that's where 19 comes, although it's not possible to have 19 separate work blocks in a month.