>>> example([1, 2, 3], [3, 3, 4, 5])
1

>>> example([1, 1, 2, 3, 4], [1, 2, 3, 4])
4

//this is python, comments if you aren't familiar
def example(list1, list2):
	count = 0
	//for every item in list1
	for item in list1:
		//if list2 contains that item
		if item in list2:
			count += 1
			//remove the first occurrence of item from list2
			list2.remove(item)
	return count