"Does convoy count as two units?" No.

Does a convoy support diplomacy?

The unit being convoyed can be supported into its destination space by any other units that border the destination space, just like any other support. However, if the convoying fleet is dislodged, it cannot convoy the unit and the entire move will fail.

What does convoy do diplomacy?

One of the more popular moves in Diplomacy is the convoy, which uses a fleet to transport an army across one or more sea/ocean spaces. The convoy's main advantage is that the army can move more than one space in a season.