When did they have time to add in more people though?
They were gonna keep spots for Bellamy, Clarke & Raven at least.
And I assume they wanted until the very last minute until they decided to not longer let anyone in.
So let's go with 3 spots they either:
1. Didn't fill those spots at all, cause waiting for them to return (which brings a whole new difficulty, probably there are tons of people standing outside the bunker door, begging to get in
2. Kept 3 spare people inside and were gonna chuck them when the rest came back?
Not likely I think