JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="numIpt" maxlength="2" />

JavaScript

function ForNumbers(evt){
    var charCode = (evt.which) ? evt.which : event.keyCode;

    if (
        //0~9
        charCode >= 48 && charCode <= 57 ||
       //number pad 0~9
       charCode >= 96 && charCode <= 105 ||
        //backspace
       charCode == 8 ||
        //tab
        charCode == 9 ||
        //enter
        charCode == 13 || 
        //left, right, delete..
        charCode >= 35 && charCode <= 46
    )
    {
        //make sure the new value below 20
        if(parseInt(this.value+String.fromCharCode(charCode), 10) <= 20) 
            return true;
    }
    
    evt.preventDefault();
    evt.stopPropagation();
    
    return false;
}
document.getElementById('numIpt').addEventListener('keypress', ForNumbers, false);