JSFiddle - React, Tailwind, and code Playground

HTML

<span class="avatar-number">12</span>

CSS

.avatar-number {
  width: 20px;
  height: 20px;
  border-radius: 50%;
  font-size: 12px;
  color: #fff;
  line-height: 20px;
  text-align: center;
  background: #000;
  display: inline-block;
}