Also, no. Pict-rs the service behind all the Lemmy image hosts, has a config of max-images=1 set by default

Lemmy also does not use the background mode it uses the active mode for uploading images to pictrs for processing.

This means that your images need to completely upload, process, and return back the url within 15/30s otherwise it fails and you get that random red box JSON error line 1.

If you scaled that out to multiple images (let's say 5), you would run into the risk of timeouts happening.
On-top of that Lemmy doesn't have that capability (yet?).

People usually upload 1 main image, then in the body of the post you can upload more images in there. Add them with the picture icon.

Click the image button under the text box

You can post a link to an Imgur album like this post did. It is not free and charges a little bit for its API but it is the most widely supported image host so some frontends have an Imgur gallery viewer built in. That's what Reddit users did before the long-overdue introduction of native albums.

However, there will likely be no preview in feeds. Look how your app handles it by visiting the user’s page: @Prismo@lemmy.world