I think it heavily depends on the company you're going to work for.
More modern Japanese tech companies may pay up to 20-25m, depending on the position. The top paying company is almost certainly Woven Planet, but Mercari, PayPay, and a few other companies may also be willing to pay that (I worked for PayPay as a point of reference here, and also was in negotiation with Woven).
If it's Rakuten or Line, you're probably going to be get offers on the lower end (maybe 12-13m). If it's Rakuten, I highly recommend asking folks outside of the company that aren't a recruiter about it before accepting.
It's worth noting that people will probably tell you that it's not appropriate to negotiate in Japan, but they are wrong. You should negotiate, and you should ideally start with a high asking, if you're going to go first (you should try avoiding going first). I recommend having your recruiter give you the rough range they are thinking, then tell the recruiter roughly what you're expecting. If you have offers from other companies, your negotiations will go considerably better, especially since you can play the "if you offer x I'll stop negotiations with everyone and sign right now" card. I was able to increase my initial offer by 60% during negotiation.
If you're unhappy with your salary (or company) after arriving, I'd recommend looking around after you get your visa, and after your probation period. Ideally you'll get a 5 year working visa. It's generally not tied to your company (though some of the visas that get you permanent residency faster are tied to your employer). I recommend looking at western companies with offices in Japan (Meta, Google, etc). They can often pay 2x what Japanese companies will pay, especially for senior engineers as the local market doesn't have a lot of senior engineers to hire.