[Share] Displaying the number of images below as a caption

First, you need to find Gallery Section ID.

In my example, it is:

section[data-section-id="66ed2c3e2f063471736e09c1"]

#1. Gallery Grid

Use this to Custom CSS box

section[data-section-id="66ed2c3e2f063471736e09c1"] {
div.gallery-grid-wrapper {
    counter-reset: css-counter 0;
    list-style-type: none;
}
figure.gallery-grid-item {
    counter-increment: css-counter 1;
    position: relative;
}
figure.gallery-grid-item::after {
    content: counter(css-counter) '/12';
    bottom: 0;
    font-size: 20px;
    left: 0;
    color: black;
}}

result

#2. Gallery Strips

section[data-section-id="66ed2c3e2f063471736e09c1"] {
div.gallery-strips-wrapper {
    counter-reset: css-counter 0;
    list-style-type: none;
}
figure.gallery-strips-item {
    counter-increment: css-counter 1;
    position: relative;
}
figure.gallery-strips-item::after {
    content: counter(css-counter) '/12';
    position: absolute;
    bottom: 0;
    font-size: 20px;
    left: 5px;
    color: black;
}}

result

#3. Gallery Masonry

section[data-section-id="66ed2c3e2f063471736e09c1"] {
div.gallery-masonry-wrapper {
    counter-reset: css-counter 0;
    list-style-type: none;
}
figure.gallery-masonry-item {
    counter-increment: css-counter 1;
    position: relative;
}
figure.gallery-masonry-item::after {
    content: counter(css-counter) '/12';
    position: absolute;
    bottom: 0;
    font-size: 20px;
    left: 0;
    padding-left: 5px;
    padding-right: 5px;
    color: black;
    background-color: #fff;
}}

#4. Gallery Reel

section[data-section-id="66ed2c3e2f063471736e09c1"] {
.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: 0;
    padding-left: 5px;
    padding-right: 5px;
    color: black;
    background-color: #fff;
}}