Number of images in Gallery Reel

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