function checkNumber(event,objThis)
{
	var eventKey = document.all ? event.keyCode : event.which;
	if(!(eventKey>=48&&eventKey<=57) && eventKey != 8 )
		return false;
}
<input type="text" size="4" maxlength="4" value="2" name="count" onkeypress="return checkNumber(event,this);">