The absolute max who could buy in this sale is the largest away allocation, so 5.5k or whatever it wa
Without being overly pedantic, 5.5k would be the minimum who qualify right? Everyone who bought Preston has got 1 away credit definitely, but more than went to Preston might have one away credit as well.
E.G Someone could have missed Norwich/Preston as credits didn't count but then used tickets from a previous season to attend Arsenal as it was a Semi. I imagine most people will have just bought all 3 though, but the number is everyone at Preston + anyone who only bought 1/2 games which could be a handful up to a few hundred extra.
Edit: But could be negated by some people having the aways but not the homes.