I’m not sure there is one. I think you can upload 5 at a time, but you can do multiple batches.
They should all be arranged into a nice little grid now as well, though I haven’t tried it on this forum yet.
Update for posterity: The
[grid]tags are a manually added extra, and won’t happen automatically
Sorry @Peitho, I borrowed one of your posts to test it on 
https://lovehoneyforum.com/t/images-to-share-in-topics/259586/480
They should get wrapped in
You can wrap them in [grid] tags like so to arrange them into the grid format:
[grid]
all the images
[/grid]