erc20.abi.json is found here https://gist.github.com/cgcardona/915852526e48e77364f75921c26245d0

balanceOf.js

task("check-erc20-balance", "Prints out the ERC20 balance of your account").setAction(async function (taskArguments, hre) {
  const genericErc20Abi = require("./erc20.abi.json");
  const tokenContractAddress = "0x...";
  const provider = ethers.getDefaultProvider("https://api.avax.network/ext/bc/C/rpc");
  const contract = new ethers.Contract(tokenContractAddress, genericErc20Abi, provider);
  const balance = (await contract.balanceOf("0x...")).toString();
  console.log(balance)
});