well, that is a good question. One I actually asked Brian once. And he didn't know, if I recall right. So I did a "test" on myself, where one Sunday morning when nobody else was around, I reset my post total to 1,000,000 (bwahahahahahaha!) but it showed up some really odd number...like 637 or something totally off of what I had put in there. But the way things are going, we shouldn't have to wait long til we see what happens once it hits 10,000. It might be a bit of a wait to see a legitimate one million post count here, tho.
[a brief pause...] Actually, the max number is 65535.