Hello, is it possible to select multiple images (around 200) and add them all at once in a UIkit 3 Gallery module? Or do I need to add them one by one?

Thanks

There is a separate gallery for this in the download: mod_uk_dgallery.zip
This allows you to display all images in one folder and you do not have to specify each image individually.

Hello, is it possible to select multiple images (around 200)

A Joomla module, or the gallery module is not made for so much images. For more than 50 pics its recommended to use a component and not a module.