I think it is pretty safe to say most folks hold one license....in the case of pricing add-ons especially.
Now...the next guessing game is how many licenses are there?
Really rough - but 2,500 to 3,000 IMHO.
The thing about add-ons is that the writers have to be patient. I would fully expect XF to end up with 10,000 to 50,000 licenses, at which point a reasonable amount of good add-ons should be able to be sold.