Here is the list of the top 10 most commond Japanese surnames. It is said that more than 110,000 different surnames exist in Japanese. But roughly speaking, one out of every ten Japanese has a surname listed here.
1. 佐藤（さとう） Sato
2. 鈴木（すずき）　Suzuki
3. 高橋（たかはし）　Takahashi
4. 田中（たなか）　Tanaka
5. 渡辺（わたなべ）　Watanabe
6. 伊藤（いとう）　Ito
7. 山本（やまもと）　Yamamoto
8. 中村（なかむら）　Nakamura
9. 小林（こばやし）　Kobayashi
10. 斎藤（さいとう）　Saito
By the way, my name is “Tanaka.” As you can see in the above list, it is the forth most common surname in Japan. When I was in the senior year in high school, FIVE Tanakas are in my class (40 students in a class)!

Here in Fukuoka Prefecture, the top 5 most common surnames are listed below.
1. 田中（たなか）　Tanaka
2. 中村（なかむら）　Nakamura
3. 井上（いのうえ）　Inoue
4. 古賀（こが）　Koga
5. 山本（やまもと）　Yamamoto