COMMISSION IMPLEMENTING REGULATION (EU) No 1079/2012 of 16 November 2012 has been published concerning 8.33 kHz channel spacing below FL195.

In short, for most of us, the rule is simple:

Article 5

Obligations of operators

1. An operator shall not operate an aircraft above FL195 unless the aircraft radio equipment has the 8,33 kHz channel spacing capability.

2. From 1 January 2014 an operator shall not operate an aircraft flying under instrument flight rules in airspace class A, B or C of the Member States listed in Annex I unless the aircraft radio equipment has the 8,33 kHz channel spacing capability.

There are no relevant derogations or exceptions.

I imagine the number of 8.33 radios required is the number of comm radios required by the applicable law (for private flights in the UK, one).