The Hermantor is one of the very few Pompey fans with some common sense and not wanting to tell the world every other sentence how amazing their support is. You're the type of fan the PST needs running the club, not attention seekers full of sound-bites living in cloud cuckoo land.
Probably because you have to be on the database to buy tickets, so there may have been 200,000 different people who are registered on the database who have bought at least one ticket in the last few seasons. It does make sense, although I'm not sure how correct that figure is.
I'm guessing, not very. At the end of last season Saints had 180,000 corporate and individual members on their database - link. Will be higher now due to new members this season but certainly 200k-250k.