// Add row to the HTML table
 function addRow() {
var table = document.getElementById('estimation'); //html table
var rowCount = table.rows.length; //no. of rows in table
var columnCount =  table.rows[0].cells.length; //no. of columns in table
    var row = table.insertRow(rowCount); //insert a row
var cell1 = row.insertCell(0); //create a new cell
var element1 = document.createElement("input"); //create a new element

element1.type = "checkbox"; //set the element type
element1.setAttribute('id', 'newCheckbox'); //set the id attribute
cell1.appendChild(element1); //append element to cell

var cell2 = row.insertCell(1);

var element2 = document.createElement("input");
    element2.type = "text";
    element2.setAttribute('id', 'newInput');
    cell2.appendChild(element2);
cell3.appendChild(element4);

//Add the cells for more than 3 columns
if(columnCount >= 2){
    for (var i=3; i<=columnCount; i++) {
        var newCel = row.insertCell(i-1); //create a new cell
        var element = document.createElement("div"); //create a div element
        var txt = document.createTextNode("cell "+i); //create a text element
        element.appendChild(txt); //append text to div
        newCel.appendChild(element); //appent div to cell
    }
}
}

// Number only text field
function numbersonly(e)
{
var key;
var keychar;

if (window.event) key = window.event.keyCode;
else if (e) key = e.which;
else return true;

keychar = String.fromCharCode(key);

if ((("0123456789").indexOf(keychar) > -1)) return true;
else return false;

<input type="text" onKeyPress="return numbersonly(this, event)">
<input type="button" value="Add row" onClick="addRow()">