Speaking of Henry's number 12 shirt - what's the official rule on "squad number" style numbering outside of international tournaments?

The majority of European teams I see do it for friendlies and qualifying games, and it's been this way for a while - I recall seeing Zidane wearing his 21 shirt in a friendly and feeling dirty (yet aroused) by the whole prospect. Is it just the home nations flying the flag for 1-11 numbering outside of tournament play?