The short answer
One to. One dō. Two fu. Forty-three ken.
Japan has 47 prefectural-level governments. Tokyo alone uses 都 (to), Hokkaido alone uses 道 (dō), Kyoto and Osaka use 府 (fu), and the remaining 43 use 県 (ken).
Why English hides the pattern
English convention normally translates all four labels as “prefecture.” That is convenient, but it erases a distinction that Japanese place names retain. 東京都 is Tokyo-to, 京都府 is Kyoto-fu, and 青森県 is Aomori-ken.
The useful memory trick
Remember the four exceptions, not all 47 labels: Tokyo, Hokkaido, Kyoto, and Osaka. Everyone else is ken.