It's not that simple.
Comparing how many ml to how many cigs is even less accurate than comparing how many cartos to how many cigs! Some juice delivery systems just suck down the juice like crazy. Some make it last. And, that's before you even take into account mg/ml and voltage.
On top of that people don't often vape the same as they smoked. Light smokers often end up being heavy vapers, and vice versa. We don't vape the same way we smoked, especially over time as we become vapers instead of smokers.
For example, I smoked about 40 cigarettes per day. But, I only vape about 2 ml per day. So, for me 1ml = 20 cigarettes. But, that's when I'm dripping on a SR 306 or 510-t. It's different in a Vivi Nova. It's different in a carto-tank. It also depends on the nic level, the flavor, PG/Vg ratio, and on and on and on.
It's just not that simple. Vaping is more complex than smoking.