I have a Drupal 7 multisite install, with five sites I run; for some reason had thought these were pretty common.
For some time, been considering upgrade to Backdrop; and I now see the process has been simplified, which is good - I'm no coder!
But not seeing info on upgrading from multisite.
I have two approaches in mind:
Create a multisite install folder for Backdrop, and upgrade sites to here, one by one.
Or, create a backdrop folder for each of the websites.
I know a number of folks do use multisites with Backdrop. I think either approach could work so the choice probably depends on your ideal outcome.