I figured out a workaround for adding/uploading more than 1 photo in a post (w the Tapatalk app).
It’s tedious, but it works: Add the photo, copy the link that appears in the post (for the photo upload), copy/paste the link elsewhere (I used Evernote), then go back into Tapatalk, delete the link/photo (which then allows you to add a different single photo) and repeat with the next photo.
If there’s an easier way to do this, it’s not easily evident to me.