How about that:
The "N" stands for naval and the "X" for the Roman number 10. That would make the NX-class the 10th naval starship class since the founding of United Earth's Starfleet.
Interesting idea. That would mean however, that every class of starship would have its own Nsomething
designation. But most vessels have the NCC, which would make all of them members of Starfleet´s 200th starship class