My Dark Before Dawn campaign LARP has a total membership of 15 with an average of 13 players per session. We have a special plot involving certain relic stones that can be expended to unlock a door into a demi-plane OR provide five exceptional successes in anything that isn't PvP-based OR to abjure an area of incorporeal beings that's roughly the size of a city mall. One of the players popped one such stone.
A couple months ago, one of the players decided his character would soon pop a stone.
I arranged for four players and five cast members to assist with the session. In the end, I increased the Cast to six and a few days before Play Day one of the puzzles meant the Good+ Ending could only be netted if there were five players so I allowed the initiating player to pick an extra person.
The main premise? There are six Riders on the Last Express and those who spend a stone may take one Rider home with them who can later be used to resurrect a recently dead mortal (no more than three days dead). Only five Riders may be selected by the initiating stone user. The sixth Rider may only be selected if all other Riders are released. So as you can see, we needed five players.
Then on the game day, two of my cast were sick. No problem. One of the benefits of keeping your GM spot clear of NPCs is that you can always pick up the slack where it happens so I took on one of the roles. When I went to draft an extra cast member, I realised that there were only three possible members of the game who weren't involved in this Special Scene at all. So I allowed seven players and nabbed one of the previously excess three members to join the Cast.
This was helped a fair deal by having this Special Train LARP (7PM to 11PM) occur after the regular main day session (1PM to 5PM). It was a long day so I made sure the Cast for the Train LARP went off to dinner with the rest of the players while I stayed back to readjust the venue from "Boozecan Pub" to "Train". It involved masking tape rooms and shifted furniture, but looked pretty good in the end.
The cast returned half an hour before the players and we managed to kick it off at 7:10PM which was pretty epic all round. The Train Players had to take a walk around the block as we weren't quite ready when they first arrived, but they were pretty happy about it.
Oh, I also managed to scoff down my burger so I'd made sure I ate.
Long story short, when you have a group of 15 players, sometimes it works all the better to find ways to include everyone rather than sending 1 - 2 people off home alone. It certainly worked out for the best this time.