Technically all Editor Extensions are bought on a per-seat basis in the Asset Store because of Unity's licensing rules. As a UI extension, NGUI counts as one of these.
That said, ArenMook has on numerous occasions said that he sees NGUI as a per company thing such that he will never pursue people for using it on multiple machines etc.
I've said it before, but from my perspective it's basically on a "don't be a dick about it" kind of license - one per company is fine. If the company splits in two, then one of them can keep it and the other should buy a new license. If someone leaves the company, he should buy his own license if he wants to use it. If two companies work together on the project, then either both should buy it, or the company responsible for the UI should just have its own - if the other company ever needs to actively use NGUI too, then they should get their own license.
TL;DR: 1 per company should be fine.
Note: Despite my moderator status, I do not speak officially for Tasharen, because I don't work here, I'm just a developer like the rest of yous, so I speak from my own perspective based on being on these forums for a long time.