FLEX-WRAP
Run 
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <style>
      * {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.flex-container {
  display: flex;
  flex-wrap: wrap;
}

.flex-item {
  height: 10vh;
  width: 20%;
  margin: 5px;
  padding: 5px;
  font-size: 32px;
}

.red {
  background-color: red;
  color: white;
}

.green {
  background-color: green;
  color: white;
}

.blue {
  background-color: blue;
  color: white;
}

.yellow {
  background-color: yellow;
  color: black;
}

.pink {
  background-color: pink;
  color: black;
}

  </style>
  <title>CSS FLEXBOX</title>
</head>

<body>
  <header>

  </header>
  <div class="flex-container">
    <div class="flex-item red">1</div>
    <div class="flex-item green">2</div>
    <div class="flex-item blue">3</div>
    <div class="flex-item yellow">4</div>
    <div class="flex-item pink">5</div>
    <div class="flex-item red">6</div>
    <div class="flex-item green">7</div>
    <div class="flex-item blue">8</div>
    <div class="flex-item yellow">9</div>
    <div class="flex-item pink">10</div>
  </div>
</body>

</html>