I've seen a few postings on this subject but no response... QandA201 summed it up best in this post: http://www.pokertracker.com/forums/viewtopic.php?f=15&t=16725&start=70#p80560
The bottom line is that the number of entrants for a tournament seems to be determined by the number of players for which there are observed hands, and even when the Tournament Histories are imported, this number does NOT get updated.
I (and QandA201) see this on PokerStars, but i've also seen this reported for iPoker, so I assume this is not site related.
FTR, I'm using PT3 Beta 26. It's really a pain to have to update these manually.
Thanks,
Pete