Seems like the info I've seen suggests a relationship between the amount of nicotine in x ml of juice at y mg strength correlates to a pack of "full flavor" cigs. Those may be accurate.
But how much juice is needed to equal 20 cigs? Not sure that can be calculated. We all have different vape habits and techniques.
I'd just fill it up, have your juice handy, and see what your experience is with your setup and methods. If it lasts all day, and you used to smoke a pack a day, then, for you, 2 ml is about a pack. Or if it only lasts half a day (or a day and a half, or...)
When I started, I was always concerned about running out of juice, running out of battery... so I never left the house with about 6 filled cartos, a couple full stardusts, and at least three batteries. Once I got used to how much I indulge, that's trimmed down quite a bit (although carrying spare everything is STRONGLY advised. Accidents and unexpexpected delays do happen, and we usually can't just pop in to 7-11 for more!)