JSFiddle - React, Tailwind, and code Playground

JavaScript

const arr = [
  { price: 10 },
  { price: 10 },
  { price: 10 },
  { price: 10 },
  { price: 20 },
  { price: 20 },
];

const obj = arr.reduce((p, c) => {
  p[c.price] = ++p[c.price] || 1;
  return p;
}, {});

const out = Object.keys(obj).map(key => {
  return { price: +key, numElements: obj[key] };
});

console.log(JSON.stringify(obj,null,2));

console.log(obj,out);