While he could easily be picking a number out of thin air to indicate "a very large number," the number twenty-seven makes some very interesting math with some other numbers we already know.
It wasn't long ago that the Governor's army consisted of 34 people--add to that four more in the form of Ben, Allen, Sasha and Tyreese and you've got 38. It's unclear whether that number includes the Governor.
You can subtract eight from that number. Why? Well, check out this trailer:
You hear that? According to the Governor, "They killed eight men back there!"
Let's assume for a minute that this number--27--is actually on the level. There will be 30 members of the Governor's army at the start of the episode, and from everything we can tell, only 28 of those will actually enter the prison grounds. Tyreese and Sasha haven't been keen to go kill a group of people including women and children already, but when you compound that by the fact that we don't see them as part of the invasion party in the sneak peek that shows Woodbury's approach to the prison, that paints what seems like a pretty clear picture that they won't be invading.
So if they've got 28 people invading--and there will be 27 deaths in the episode? That sounds like a real bloodbath. Especially when you consider the fact that the main fighters for the prison appear to be gone when Carl, Carol, Hershel and Beth are seen packing up their belongings. Long-standing rumor, stemming from local reports, had Rick, Daryl and Michonne (at least) attempting to penetrate the Woodbury gates in the finale, meaning there's a pretty good chance that they'll be coming back up behind the Governor and his men once they realize that the most vulnerable members of their group are at risk.
Now, obviously most of the deaths were always going to be Woodbury's, and most of those will be characters whose names we don't eve know--but it's worth noting that with the numbers matching up so closely, it looks as though almost everyone in the Woodbury group could be gone by the end of the season. What that would mean for that group's future, of course, is hard to say--and neither Tyreese nor Andrea would like very much to see them so vulnerable, so it could be interesting to see whether one of them stays behind to help Woodbury get back on its feet.