You have to iterate through the filter object somehow. One way is to convert it to a list:
l = list(filter(lambda x: x > 3, n))
len(l) # <--
But that might defeat the point of using filter() in the first place, since you could do this more easily with a list comprehension:
l = [x for x in n if x > 3]
Again, len(l) will return the length.