So you actually count by segments of the finger, using your thumb as a tapper. Crook your index finger and look at it - three segments, right? (At least for most people) The segment nearest your hand is 1, middle is 2, end is 3. Middle finger is 4, 5, 6, etc. So you can count up to 12 on four fingers.
Then you use your non-dominant hand to mark dozens. Count to 12 on your main hand, raise on finger on your second hand, repeat. So you can actually count to 60 just on your hands.
So you actually count by segments of the finger, using your thumb as a tapper. Crook your index finger and look at it - three segments, right? (At least for most people) The segment nearest your hand is 1, middle is 2, end is 3. Middle finger is 4, 5, 6, etc. So you can count up to 12 on four fingers.
Then you use your non-dominant hand to mark dozens. Count to 12 on your main hand, raise on finger on your second hand, repeat. So you can actually count to 60 just on your hands.