Is there a way to import several tracks at once from my Boss BR8 so that they show up as separate tracks in GarageBand?

If you can get them into iTunes, or in a Finder window, you can drag into Garage band from there.

I had to google your device - looks like it's a vintage multi-track recorder that records to Zip disks. So I guess there's no digital USB or Firewire multichannel output and no way to transfer your tracks digitally - or is there? If not, your only option is to record the device's audio track by track into GB.