Sum of all counts in a collections.Counter
The code you have adds up the keys (i.e. the unique values in the list: 1+2+3+4+5+6=21).
To add up the counts, use:
In [4]: sum(Counter([1,2,3,4,5,1,2,1,6]).values())
Out[4]: 9
This idiom is mentioned in the documentation, under "Common patterns".
Sum the values:
sum(some_counter.values())
Demo:
>>> from collections import Counter
>>> c = Counter([1,2,3,4,5,1,2,1,6])
>>> sum(c.values())
9