Do you need multiple image_teaser files, to create a new record for each one? Or do you have multiple different fields in a single record that you need to upload for all at once (e.g. a teaser image and a banner image and a footer image)? The solution is quite different for those two scenarios.
Also, can you share what this part of your form currently looks like?
I would think that each of the images would result in a separate record being created in some other table, and you’d do validation of them on a per-record basis.
You’ll need to make a custom validation function for that, then, though you could perhaps call the underlying code for uploadedFile from your function.
Can’t display a message below the field error field, probably because it’s called image_gallery[], please how do I display an error message for a given field?