I think it's 9 fleets... as convoy will go via shortest route possible. If you are convoying from Syria to St Petes then the fact that fleets in say Irish Sea/Gulf Of Lyon also order a convoy doesn't dictate if it will suceed or fail so don't think you can include them.

But yes, it can be done. It makes a real mess of the map with all those lines going across the middle. I've done it recently, but I can't remember what game it was in!

http://www.phpdiplomacy.net/mapstore/301/3011/29-largemap.png
One example

I think your examples counts as 8 figlesquidge: the Aegean and Irish weren't needed, so shouldn't count.
Instead of NAO you could take it through the channel and the north sea for a genuine 9.
Or go if you are headed for Swe or Kie: meander around the north of England for a 9, but not 13 the way Bravehart and I count it.

16
The Fleet length depends on the house rules.
In many of the early Judge Automatic Adjudication house rules the convoy had to have a specific fleet route and if any fleet along that specific fleet route was dislodged the convoy would fail.
Therefore you could have a convoy from StP that included ALL
continuous sea spaces regardless of the most efficient route
so for example the army convoy would be something like
A StP to Syria via
Bar-Nwg-Nth-Ska-nth-hel-nth-eng-iri-NAt-Mid-Wes-Lyo-Tyn-Ion-Adr-Ion-Aeg-Eas
This would use every Sea Space on the Board except for Bot-Bal-Bla

The exact route decision in the house rules of the early judges was driven by the need to simplify code and to eliminate the unwanted convoy and certain paradox's that could come from multiple convoy routes as was introduced in the standard rule book around 1981 or maybe 1976 .
Edi

Edi to the rescue! :D

How could you use Heligoland? It would just loop in and out of North

The point is that in the rules on the judges (and in the English FtF tournament) you have to designate the route and can have a loop.
There could be a diplomatic reason for a loop, for example an alliance of Germany/England have agreed that they want to convoy army Denmark to Belgium, but for some reason the English are worried that the German fleet in Helgoland may do something vile and if that happens he wants to keep the army in Norway. so as a safety measure, he tells the German that he is convoying through Helgoland as his route. Now if the German does not issue the order then the convoy does not go through and the English army remains in Norway.
This is how you learn to use House Rules, no matter how odd, to support your negotiations and make them work for you in little fail-safe methods that reduce the number of surprises that you can be exposed to.

Yeh sorry, I have a longer one, but that's been lost when the game saving style was updated. In that game we've also got the longest line, going smyrna to Clyde or something

Very cool Convoy image, figle!

If your allowed loops then is it possible to use all 19 sea territories?

No. Unless you're also allowed to convoy through Con and Kie.

I didn't think you could convoy through land. You can't.

in an alliance game on FB i convoyed from con-St P and the next move from St P to Smy

http://apps.facebook.com/phpdiplomacy/board.php?gameID=2949&msgCountry=Russia

maniac that link doesnt work it wouldnt let me toggle between moves

sorry whiteSammy - you'll just have to trust me :)

i can see the ships lined up
im just pointing out that you cant view the history of facebook games