A world cannot be conquered from orbit, no matter how powerful the attacking fleet is. To take over a world, infantry legions must land on the surface and take over the command centers and central governments so that order can be maintained after the battle. Clearly then, the primary goal of the fleet commander in attacking a planet will be to land infantry on the surface.
Conversely, because troops are most effective at fighting troops, the defender must have enough legions to repel the invaders. In general, all other things being equal, the attacking force must be larger than the defending force, as the defenders will have the advantage of stationary weapons and fortifications.
Normal infantry legions are simply enlisted from the population of each world. The nominal awry size for a world is dependent on population and type. Agricultural worlds may have 0.05% of their population in the military, while a capital may have as much as 2%.
Sometimes worlds and empires will use genetically engineered humanoids as either defenders or attackers. These troops are called "elite infantry"; they are much more powerful than normal humans because they have been genetically created to be both stronger and more cunning. Since part of the creation process involves ambrosia, worlds that produce elite infantry must have a constant supply of the drug.
Worlds of type elite academy will produce elite infantry as a resource.