效果图

 <div class="applyInput cb mt3">
    <textarea placeholder="请输入" maxlength="500" @input="descInput" v-model="desc" />
    <span class="numberV">{{txtVal}}/500</span>
 </div>

<script>
export default {
  name: 'applyAgent',
  data () {
    return {
      txtVal: 0,
      desc:""
    }
  },
  methods: {
    descInput(){
        this.txtVal = this.desc.length;
        }
    }
}
</script>