1N, 2N, 3N devices are well defined
but 4N and 5N and 6N parts were never?

I believe the initial number is the number of PN junctions
in the device -- at least as far as junction devices go.

** How many junctions does a 2N5459 j-fet have?

Technically, one. JFETs are not generally considered "junction" transistors,
though they contain a junction. That's why I phrased my remark as I did.