Booking accomodation when in Japan

My friends and I will be travelling around Japan for 2 weeks visiting Tokyo, Kyoto, Osaka and possibly Hiroshima, Nagasaki, Beppu, Iga Ueno, Yokohama and Nagoya (a lot I know).

Was wondering if its easy to go to the cities and just walk into a hostel and ask for a room or 2. There are 7 of us, we dont mind being split up if need be. Do you have to pay extra at some places if you get a room that day? We are going to be there from late january to early february, is this a busy time?

It is not a busy time but I strongly recommend you to book in advance. Good (and cheap) hostels will go first.

It depends, with 7 people the day of arrival some places might some places might not.

If you have a good idea already of the dates of which cities you will be in, it is always good to book in advance, for piece of mind that you know you will get a room.

Some cities you mentioned you can just stay in one places, aka, Kyoto and Osaka are near each other so you can just stay in one city your entire time and visit the other city from there, same for Tokyo/Yokohama they are very close to each other you can stay in Tokyo and day trip to Yokohama.