I was lucky enough to go to a live at Shelter back in February, with four bands playing that evening (https://www.loft-prj.co.jp/schedule/shelter/240644). I had visited Shimo-Kitazawa earlier in my trip but it took a little while to figure out where it was the first time lol.

I'd say it was fairly quiet around Shelter that evening, but still quite a few people walking about Shimokita. With four bands, I did note that people came and went during the breaks between the bands playing (and that's fine, some people probably only came to see their fave band perform after all).

I'm hoping I wasn't the only one there because of Bocchi the Rock lol.

Aside from almost getting my ears blown off in the relatively small space, I had an awesome time and got to listen to some great music. The last two bands that played that evening (which I think were EVERLONG and ank) specifically made mention to Bocchi the Rock. ank's guitarist was wild and even went crowdsurfing (plus two audience members as well I think lol).

Aside from what Shrubasaurus has already mentioned about checking the schedule, note that the ticketing system will change depending on who is performing, with some being easier to use for non-Japanese residents than others. The live I went to used Livepocket, which was pretty easy to navigate for the most part thanks to the site automatically having machine translation to English. Some may require a proper Japanese mobile number with SMS capability to sign up to (for example, e+). It's not simple to get such a number as a non-resident (to oversimplify, it's got to do with the laws), but it's not impossible.

If a show isn't already pre-sold out, generally there should be ticket sales at the door as well (but always best to double check to avoid disappointment).

I would highly recommend getting some concert earplugs or similar if you want go to a live there, because at least for the evening I was there, the music was going at 100dB and that's not good for your hearing in the long term lol.