To add number of images in Gallery Reel Squarespace, you can follow these steps.
First, find Reel ID.
Next, use CSS code like this
section[data-section-id="62309c7df0e62e3ac884c504"] {
.gallery-reel-list {
counter-reset: css-counter 0;
list-style-type: none;
}
figure.gallery-reel-item{
counter-increment: css-counter 1;
}
figure.gallery-reel-item::before {
content: counter(css-counter) '/12';
position: absolute;
bottom: 0;
font-size: 20px;
left: 20px;
color: black;
}}
Result