Hi Edward,
Film (especially Fuji film!) is readily available in Japan, in all formats, although you would have to go to specialist places to get sizes larger than 120. The best places to buy film are:
Yodobashi Camera (Tokyo, Osaka)
Bic Camera (Tokyo, Osaka, Kyoto, everywhere)
Kitamura Camera (in smaller centres, smaller stock, mostly digital but still has some film stuff)
Yodobashi and Bic Camera are usually located near train stations in all major cities. Google maps will help you pinpoint the locations. The Shinjuku branch of Yodobashi is the Mecca of film photographers around the world (the photo stream I linked is one of many). Be warned though -- Yodobashi in Tokyo is not one store, but spread over multiple buildings. The film store is in one of the smaller buildings, and the film will be on the main floor of the building. Upper levels will have other kinds of (film) gear (tripods, lighting, etc).
You don't say where you are located, but film in Japan is a bit more expensive than the US, but about the same as Canada/Europe (give or take). I wouldn't worry about x-rays at all (I've flown in and out of airports with up to 10 scans in one trip with no problems). I guess buying film in Japan will depend on how paranoid you are about it. I live here now, but when I flew here for vacation I brought some film with me from home to get me started, and bought the rest in Japan. Film is kept in coolers and is open to the customer -- there's no asking for it because it's locked behind the counter somewhere. You just take what you want and bring it to the counter. Expensive items (sheet film and often 5-roll (or more) boxes of 120 film) tend to be empty -- you take the box (or the paper) to the counter and they'll give you the film when you pay for it.
As for the number of rolls, that'll depend on what you like to shoot. When I bring two formats, I usually bring enough for two rolls a day overall, with 2/3 going to 120 and 1/3 to 35mm, and about the same split in black and white and colour. But I also consider high profile places where I'll probably want to shoot multiple rolls a day, and days where I'll probably shooting very little. It usually increases my two-a-day estimate a bit. YMMV. Again, Japan is not a place where you need to worry about film -- it's readily available, it's cold-stored, and it's fresh. It may be a little more expensive, but you can get it -- so don't worry about running out.
By the way, welcome to APUG!