I would assume that it would at least get the next two. But it might be more. The reason I say this is because the GNex is a dual core OMAP. I think the processor speed and type are the biggest factors in which phones get updated. I don't think there's too much that Google could add to Android that a dual core processor wouldn't be able to handle.
Depends on what is required then. The Nexus One isn't getting ICS. I assume this is hardware related. So I think it really depends on how fast hardware has accelerated 2 years from now and what would be the minimum requirements to run the new OS versions. Who knows. But I don't think you should based your decision to buy now on what may or may not happen in 2 years. Google probable doesn't even know what will be required in 2 years.
No. I think the OP is asking about future OS version updates like GB ---> ICS. I don't think this Nexus will get the next OS version before the next Nexus device and so and so forth.
I posted that because it's a moot point question as there aren't any phones that will be updated more than this... so what's the point of continuing the thread.