The rule for daemonic ritual does not specify.
The rule for understrength units says:
Each unit's datasheet will describe how many models make up that unit. Sometimes you may find that you do not have enough models to field a minimum-sized unit; if this is the case, you can still include one unit of that type in your army with as many models as you have available.
If yo uare using points, you must still pay the points cost as if you had a minimum-sized unit, even though it contains fewer models. an understrength unit still takes up the appropriate slot in a Detachement
So basically you can summon 1 understrength unit in your army of a type.
I am wondering now if you can summon say an understrength Bloodletter unit if you have already a unit of bloodletters on the battlefield. As the rule says " you can still include one unit of that type in your army"