First, I would recommend adding a smaller size and eliminating the largest size. If your espresso is good, people who want Americanos will want the smaller options. Always.
Based on your question, I would recommend 2 shots for the 12 and 3 shots for the 16. I would also do 2 shots for an 8, and not consider the 20.
For us, we do everything std. double shot because those who want larger are not really interested in the espresso, and can add shots if they like.
I agree with John.
Depending on your espresso, 8-10 oz is a pretty good double americano. Scaling up from there, your 12 would be 2 or 3 shots, 16 would be 3 or 4, and a 20 (if you offer it) would be 4 or 5 plus a signed waiver from the customer's cardiologist.
Please note that this is based on the notion of a .75-1.0 fl oz shot, a pair of which have been extracted from 16-18g of freshly ground coffee in an appropriate time. Smaller or larger shots or baskets will change the equation.
When done properly they are delicious, though most places make them far too weak (a good thing, given the taste of many of their espresso.)