I think the easy way to do this would be:

<input type="text" onkeypress='return event.charCode >= 48 && event.charCode <= 57' />

But the problem comes up when you paste some text then HTML5's number type input may be a better choice:

<input type="number" />

BTW you can find better suggestions if you search the SO like this.

This seems hacky, but does the trick:

function digitKeyOnly(e) {
  var keyCode = e.keyCode == 0 ? e.charCode : e.keyCode;
  var value = Number(e.target.value + e.key) || 0;

  if ((keyCode >= 37 && keyCode <= 40) || (keyCode == 8 || keyCode == 9 || keyCode == 13) || (keyCode >= 48 && keyCode <= 57)) {
    return isValidNumber(value);
  }
  return false;
}

function isValidNumber (number) {
  return (1 <= number && number <= 10 )
}

<input class="text" name="Serial_Num" type="text" id="SrNo" size="2" maxlength="2" onkeypress="return digitKeyOnly(event)" />

Match against /^\d+$/. $ means "end of line", so any non-digit characters after the initial run of digits will cause the match to fail.

Edit:

RobG wisely suggests the more succinct /\D/.test(z). This operation tests the inverse of what you want. It returns true if the input has any non-numeric characters.

Simply omit the negating ! and use if(/\D/.test(z)).

Simple way to check that enter value is numeric is:

var checknumber = $('#textbox_id').val();

    if(jQuery.isNumeric(checknumber) == false){
        alert('Please enter numeric value');
        $('#special_price').focus();
        return;
    }

The first character is unrestricted because you have nested keypress handlers. Try this:

$('.Number').keypress(function (event) {
    var keycode = event.which;
    if (!(event.shiftKey == false && (keycode == 46 || keycode == 8 || keycode == 37 || keycode == 39 || (keycode >= 48 && keycode <= 57)))) {
        event.preventDefault();
    }
});