You don’t need to use the “Add more” link. In fact, it is not used att all in the “Image carousel” element.
To add multiple images, all you have to do is click on the “Choose Image” button and once the list of available images opens, hold down the CTRL button and start selecting multiple images. Once you are done selecting the desired images, release the CTRL button and click on the blue “Use Images” button.
I have attached a few screenshots so you can see the procedure.