The answer is simple, 1.
You're best to use the term bundle instead of license. The best value for purchasing NX is buy a bundle which packages together functionality at a cheaper cost rather than buying the individual features (like solid modeling, translators, drafting .etc).
IN terms of the licensing/how they can be used, there are node locked and floating bundles. My recollection is there are 3 node locked, the rest are floating.
There's no restriction on the minimum number of bundles you can buy, so you could buy 1 bundle only to do what you want.
The choice of how many you need, would be determined by the number of concurrent users you will have using NX at any point in time. So if you have to have 3 users working on NX at the same time, buy 3 bundles.
In that example, the license you would buy would be a license file which contains 3 'floating' bundles Your license server would have a license file, coded to serve out 3 floating bundles (of whatever type you need) to the users.
Anthony Galante
Senior Support Engineer

NX3 to NX11 with almost every MR (24versions)