<ul class="pictures">
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
  <li><img src="https://picsum.photos/200" alt=""></li>
</ul>

body {
  padding: 30px;
  font-family: sans-serif;
}

* {
  box-sizing: border-box;
}

.pictures {
  counter-reset: images;
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
}

.pictures li::before {
  counter-increment: images;
  content: counter(images); 
  position: absolute;
  top: 20px;
  right: 20px;
  background-color: #fff;
  padding: 5px;
  border-radius: 5px;
  display: flex;
  justify-content: center;
  align-items: center;
  opacity: 0;
  transition: all 0.5s;
}

.pictures li:hover::before {
  opacity: 1;
}

.pictures li {
  position: relative;
  padding: 10px;
}