How tall does the packet say the plants will be and how big are the containers? That would determine how many plants you need but your estimates seem OK. You may have to experiment a little.
I have grown different types and varieties before. The one thing about nicotiana is they stay very small for a while. They will stay small if you leave them in the small starter cells. They need to be transplanted into the larger, deeper pot first and then they take off. So what I usually do is start them inside. Once I can handle them, I transplant several of them into an 8" pot left over from the fall mums. At some point the leaves will enlarge and that's when I separate them.
