SOURCE

<div class="parent">
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
</div>
<div class="parent">
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
</div>
<div class="parent">
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
  <div class="child">
  </div>
</div>

.parent {
  margin-bottom: 10px;
  width: 200px;
  height: 150px;
  background-color: black;
  display: flex;
  flex-flow: row wrap;
  align-content: flex-start;
}

.child {
  box-sizing: border-box;
  background-color: white;
  flex: 0 0 25%;
  height: 50px;
  border: 1px solid red;
}