Hello tita -
Please be aware that the template selector requires you to select the number of images when you add a template if you have not added your images to the listing first. If you have created Item Details profiles, as it sounds like you have, and added a template with 24 images selected, that should resolve any difficulty like this for your listings posted after the profiles have been updated (updating your profiles would not affect already active listings using them as they posted with the prior version of the profile).
If you have posted listings since making the change you describe and are still having difficulty with the images in those listings, please let us know in a support case which listings are encountering this issue so we can have a look into it for you.
For listings posted prior to the change you refer to, unfortunately, yes, to update the images in the listing you would need to edit them individually.