Limit the amount of times a smart contract can receive ether

Question

How would I set a limit to the amount of people who could send ether to a contract? I can't find anything on google or youtube. Ideally the contract would stop taking inputs after 2(or however many) different people have sent an amount.

Below is simple code from https://solidity-by-example.org/sending-ether/ for sending ether to an address, but i'm not sure what I would do. I'm using remix and running it with ganache on my machine. Is it possible to make it read-only after X transactions?

https://github.com/CrazybutSolid/ethereum-lottery/blob/master/contracts/lottery.sol has a kind of similar example but it seems too complicated and I'm still a noob.

pragma solidity ^0.5.16;

contract Receive {

    function () external payable {}

    function getBalance() public view returns (uint) {
        return address(this).balance;
    }
}

contract Send {

    function sendViaCall(address payable _to) public payable {

        (bool sent, bytes memory data) = _to.call.value(msg.value)("");
        require(sent, "Failed to send Ether");
    }
}

Thank you

Answer 1

If you want to restrict it to at most 2 transfers, then you can do:

uint256 public count;

function () external payable {
    require(count < 2);
    count++;
}

If you want to restrict it to transfers from at most 2 different senders, then you can do:

uint256 public count;
mapping (address => bool) public senders;

function () external payable {
    if (senders[msg.sender] == false) {
        require(count < 2);
        senders[msg.sender] = true;
        count++;
    }
}