It depends on the level of organization and communication. A Roman Legion,
for example, was somewhere around 6000 (I'm using the figures from The Glory
of Rome), plus auxilliaries. Several legions could be placed under a single
commander. Granted, no Cerilian nation has the resources to do this sort of
thing.
Also, nothing says that an invasion force need be under one commander. Say
Boeruine invades Taeghas and kicks off the big war with Avanil. The Duke's
knights, scouts, and other cavalry units sweep south under one commander
through Portage, Bayside, and Seamist. The Duke's infantry is divided into
two commands, to hold Portage and Wilder's Gorge; one command also keeps an
eye on Rhuobhe. Finally, a third force sails from Seaharrow and Tariene to
land troops at Stormpoint. One invasion, four commanders--all orchestrated by
the Duke.