Comprehensive guide on changing the photoset layout on your Tumblr theme. By default multiple photos in one post show as a carousel, with a number photo counter and arrows to cycle through each. The options below will show you how to change it so photos display one after another using Stash Tumblr Themes.
New Themes
Look for the option “Photoset Layout” then switch it from Carousel to Normal. This works for new posts after Tumblrs NPF update around 22/23.

Using CSS – Works on legacy posts also
Same can be applied by using Custom CSS, this method works for older posts and will hide the icons over the photosets and display them one after another.

On Customizer scroll and hit Advanced Options. Inside the Custom CSS box copy/paste the code below in bold:
Hide Photoset count
To remove the photoset counter that shows in the box over photosets simply copy and paste the code below in to Custom CSS which is in Advanced Options.
To hide just the numbers:
.post-photoset .photoset-count {
display:none !important;
}
To hide the numbers and arrows:
.post-photoset .ps-options {
display:none !important;
}
To show on hover only:
.post-photoset .photoset-count {
opacity:0;
}