Number of players used this season will be one (for reference, we had used one of the lowest at 23 last I saw (L1 average was 29, albeit the January will have skewed some numbers slightly)).
How many of those all those minutes played will have been played by the first XI.
Depth of squad. Portsmouth or Sunderland can probably mix 5 or 6 of their players up if needed, we can probably get away with 2 or 3 before we see a drop off and in some areas, there's a massive drop off with just one change (Hopper, for example, or Bridcutt)).