Hello,
Cranium Preserver knows one version of the create undead ritual for every four heads it has preserved (a typical cranium preserver has 12 preserved heads). The cranium preserver can use its heads to make separate rolls as both the primary and secondary casters of the ritual.
Undead created by the cranium preserver are they under its control ? (4 minions max ?)
Thanks for your future answer.
The Cranium Preserver doesn't seem to say that the ritual works any differently than normal for them, so it depends on the result of the Primary check.
If the question is about specific creatures that exist in the context of the adventure that thing is published in, that's a matter of "what does the adventure present as the current situation?" Not a question of creature rules.