You can simply use regex to replace all occurrences in a string.
$(".Number").each(function() {
$(this).text($(this).text().replace(/Monster/g, "Witch"));
});
$(".Number").each(function() {
$(this).text($(this).text().replace(/Girl/g, "Boy"));
});
See a working JSFiddle here.
Note: Keep in mind, you can't have two elements with the same IDs (denoted by '#'). So, I changed the attribute and selector to class.
If you only want to change the value on one element, you can move the logic from the second block to the body of the first:
$("#Number").each(function() {
$(this).text($(this).text().replace(/Monster/g, "Witch"));
$(this).text($(this).text().replace(/Girl/g, "Boy"));
});