I'd guess that Satoshi has 10,000 - 50,000 BTC. He once said that he has plenty of BTC, but he didn't seem to be mining much.
I'd be surprised if he has less than 50k. I mean there must have been a whole year there with less than 50 miners. If he even kept 1/50th of a years worth and no others that's 50k.
For those who don't realize, difficulty was 1. When I came along it was jumping from 45 to 210 and it stayed reasonable to solo mine on a CPU for a while after than. Bitcoin had already been chugging along for about 18 months at that point.