I have read quite a few articles about Satoshi owning around 980,000 bitcoins which he mined during the early days of bitcoin. However, none of them are sure about his bitcoin address. And till date I haven't seen an account with that many bitcoin holdings. So it may not be true entirely. But truly I feel that Mr. Nakamoto deserves to be a billionaire for his path braking innovation and should have given Noble prize for creating an alternate economy which can't be controlled by anyone. So whenever we will talk about free economy in future. Mr. Nakamoto will definitely be highly regarded for creating bitcoin, a decentralized currency for everyone.
Obviously he didn't use a single address for mining, instead he used a new address for each new block. Due to this it's hard to tell for sure how many coins did he mine because there may have been some other early miners. The theory that Satoshi owns around 1 million coins is based on alleged pattern in mining that can be noticed in the first year of Bitcoin mining.
If you want to read the full theory, you can check it here -
https://bitslog.wordpress.com/2013/04/17/the-well-deserved-fortune-of-satoshi-nakamoto/