i kinda don't understand. why tokyo have 3 but miyagi (karasuno prefecture) only 1?

It's because there are waaay more high schools in Tokyo. So it's decided not just by prefecture, but by population as well.

The larger prefectures with significantly more teams like Tokyo and Hokkaido get 2 representatives. Also, the prefecture hosting the Nationals always gets a 'host representative' which is why Tokyo has three.

oh... i see. tx for the info