Edit in JSFiddle

<div class="row">
  <div class="block block--2">1</div>
  <div class="block">2</div>
  <div class="block">3</div>
  <div class="block">4</div>
  <div class="block">5</div>
  <div class="block">6</div>
  <div class="block">7</div>
  <div class="block">8</div>
  <div class="block">9</div>
  <div class="block">10</div>
  <div class="block">11</div>
</div>
* {
  box-sizing: border-box;
}
.row {
  display: flex;
}
.block {
  height: 100px;
  background: pink;
  border: 1px solid green;
  flex-shrink: 0;
  flex-grow: 1;
}
.block--2 {
  flex-basis: calc(100% / 12 * 2);
  max-width: calc(100% / 12 * 2);
}