@freeplay This issue has been resolved in my Mastodon fork (Fedibird).
According to Misskey, it allows up to 16 (or less than what is specified in the server settings), after which it will display that number and a link to the original post.
The WebUI look could be a little better.
Most client applications support 5 or more images and display them well.
I'd like to open a pull request to the mainline someday.