OakR is correct: the number of pilots under you in the SYSTEM determines less than the number of pilots under you in your BASE, at least when it comes to schedule stuff. As things stand right now, going from the last guy in MCO up to about the 12th guy from the bottom spans almost 200 seniority numbers. That is, bottom guy has 146 beneath him in the system and 0 beneath him in MCO; Mr. X has 12 beneath him in MCO and about 325 beneath him in the system. At MCO, in July, that Mr X can hold a hard line, although he's flying a lot of weekends.
At a more "commuter" base, at 12 from the bottom, you're holding reserve, reserve, reserve. At the bases with more pilots who live there, reserve tends to go more senior... get paid to hang out at home, and pick up open time on your days off. I was surprised how LITTLE reserve went to the bottom of the seniority list in MCO... mostly blank lines, which can be hard lines, if lots of guys have vacation or training that month, or reserve, if not.
If you have more questions, feel free to ask!
Snoopy