Edit in JSFiddle

const goods = [{
  id: "1",
  goods_name: "水立方",
  goods_price: "30.00",
  goods_num: "15",
  count: "0"
}, {
  id: "2",
  goods_name: "农夫山泉",
  goods_price: "28.00",
  goods_num: "10",
  count: "0"
}]

var app = new Vue({
  el: "#app",
  data: {
    list: goods,
  },
  methods: {
    addCart(item) {
        item.count++;
      },
      lessCart(item) {
        item.count--;
      }
  }
})

<div id="app">
  <ul>
    <li v-for="item in list">
      <p>{{item.goods_name}}</p>
      <p>{{item.goods_price}}</p>
      <a v-show="item.count > 0" @click.stop.prevent="lessCart(item)">-</a>
      <input v-show="item.count > 0" v-model="item.count">
      <a @click.stop.prevent="addCart(item)">+</a>
    </li>
  </ul>
</div>