It was only Nael's legion which was loyal to him. Because he was some sort of spiritual leader.
http://lodestone.finalfantasyxiv.com...6d5767ae15c7b3
The system in which each legion is divided is pretty specific...
And if we know 2 legatus, and each has 1 legion, it wouldn't be crazy to say (regardless of the word they are using to refer to them) that its 1 legatus = 1 legion.
And since SE posted there are 14 legions, we are deducing each has one leader. Thus, 14 legatus.