I use `jj log -r ..` for that, which is just an open ended range. It's not the "15 more" but it's what's worked for me. I suspect you could do it with some sort of revset stuff, but I like to keep it simple.

Easy: `jj log -n 25`

(Default is 10 iirc, so if you want 15 more... 25)

If you want everything, ever: `jj log -r ::`

Or every ancestor of your current change: `jj log -r ..@`