Best way to send multiple images through API: one call per image or one call for all images

I´m debating between these two options:

• Sending multiple images in one API call

• Use one API call per image

I need to send at least 15 images every time.

What is the best option in terms of API design? what are the advantages and disanvatages?

I´m using FastApi and the images are being sent inside a json body codified in base64

Thank you in advance!

I tried both but the answer is not clear to me