Actually the contract with the GDS (Global Distribution System) dictates how many seats per fare bucket are displayed.

The options are 4, 7, 9, or NO participation at all.

If you look up Delta on Sabre, you will only see "Y4B4M4.." etc but Delta on Worldspan may show Y9B9M9...

The # of seats may not reflect actual inventory. I teach agents to always "IR" the record to make sure the seats come back confirmed... sometimes it may way Y4B0M0... and you grab 4 "Y" seats, the SS message can come back unconfirmed, meaning no seats are open.