How many people will the largest sealing room hold?

I would call the Temple for that information.

Temple will give you a strict upper limit for the East Sealing Room. Don't exceed it by more than a few I agree it is 40ish.

40ish

As others have said, maybe 40 seats, but standing room also for 50 or so.

40-50ish

Like 40 to 50 but some would be standing.