Where to find information about number of seats knowing the aircraft Registration ID?
The usual way would be to find out which airline operates that aircraft. Usually Google will give the answer. Otherwise planespotters.net will help. In this case it is Norwegian.

Then seatguru will show you what the interior configuration is. 

I don't know whether these website have public API's that could help you and I think that is a bit off topic for Aviation.SE.