<ul>
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
  <li>6</li>
</ul>

ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
  display: inline-flex;
  flex-direction: column;
  height: 3em;
  flex-wrap: wrap;
  align-items: flex-start;
}