I have the code like this in the functions.php file.
add_image_size("slider", 527, 350, true); // 1st thumbnail
add_image_size("slidertop", 120, 80, true); // 2nd thumbnail
add_image_size("topweb", 60, 60, true);// 3rd thumbnail
Meaning on every image upload via WordPress standard upload feature
WordPress creates three thumbs for me.
POINT: For gallery images I just need one thumbnail.
Does ACF Gallery have the option to suppress not needed thumbnail sizes when generating the thumbnails.
I basically have 100 galleries and over 10.000 images and creating extra thumbnails is not smart in my case.
I checked but WordPress seams not solved this problem yet.