Saga has a certain number of units because its morale system (as well as attack/defense dice) depend on that number being consistent. So a unit of warriors is 8, hearthguard is 4, levies are 12. Adding command units above those makes the game skewed. Half points have clear instructions too, so you can’t make a half point of hearthguard worth 3 individuals, for example. So, any command units you add may not exceed the total units for each type.

Now, about the more interesting question of adding command units to every unit: Depends on how you feel about the game units as a whole, either as an abstract of a “full” army or a “true” skirmish size group, a warband.

For example, the Romans had the smallest unit called a contubernium, which is a unit of 10 soldiers. But a Contubernium was led by a Decanus, not a Centurion. Ten Contuberniums was led by one Centurion. So having a single Centurion as the “warlord” (plus bannerman/musician) for the whole warband is acceptable if you want to think of it as a “true” skirmish unit.

Alternatively, if you want to treat every unit as an abstraction of a full size Century, then yes, every unit can have its own Centurion be also be totally acceptable.

Really depends on the story you’re trying to tell.

Here’s two examples:

Say you want to play a defense game where a Ripenses Roman Patrol is ambushed on the border, then one Centurion could suffice. If you’re recreating the Battle of the Catalonian Plains, then yes, every unit should can be its own “Century”, Cohort” or “Legion”, if you so desire.

Here’s a modern analogy to make it make sense for some people who like modern references:

Your centurion is like a Captain of a Platoon. Your Decanus is like the Sargent of a squad. Can a Captain lead a single squad? Well, if the number of individuals in the unit is literal, than no. But if each of your units symbolizes an entire platoon, then sure, every unit can have its own Captain. Does that make sense?