[Vuejs]-Set Index in computed function

Try this

If you want to pass the index from the loop to Autofill, then you should use the methods.
You cannot pass data to a function from computed.

<div class="inputArea mt-2" v-for="(id, index) in inputs" :key="index">
  <div class="row">
    <div class="col-md-6 m-1">
      <div class="mt-2">Number</div>
      <b-form-input type="number" v-model="Number[index]"></b-form-input>
    </div>
    <div class="row">
      <div class="col-md-5 ml-1 mr-1">
        <div class="mt-2">Autofill 1</div>
        <b-form-input type="text" :value="Autofill(index).autofill1" </b-form-input>
      </div>
      <div class="col-md-5 ml-1 mr-1">
        <div class="mt-2">Autofill 2</div>
        <b-form-input type="text" :value="Autofill(index).autofill2" </b-form-input>
      </div>
    </div>
  </div>
</div>

<script>
export default {
  //...
  methods: {
    Autofill(index) {
      var returnelement = {};
      if (this.json != undefined) {
        this.json.forEach(element => {
          for (const item of index) {
            if (+element.number === +item)
              returnelement = element;
          }
        });
      }
      return returnelement;
    },
  },
}
</script>