question on N routers and N adapters

I understand that N routers need multiple antennae in order to use MIMO. (let me know if I'm wrong on this). Therefore do most N routers have multiple antennae? If so how do you know how many antennae they have since I think some are internal?

how about the N adapters? I think I saw some desktop internal N adapters with 2-3 antennae. How about the N USB adapters? If they only have one antenna can they still do MIMO? Or by virtue of the fact they only have one antenna they cannot do MIMO?

Also what happens if you have a MIMO capable N router and a non MIMO adapter? Does some type of MIMO take effect or is it a non MIMO world then in that scenario?

Thanks. I'm trying to learn!