For example, let us say 12345 is a descendent concept at a level higher than 0. Can this descendent role up to mutliple ‘mother’ concepts of 0 or just 1 concept of 0?
I’m not sure I got your question. Min/max levels of separation isn’t an absolute scale of hierarchy but a relative one: 0 indicates that this is an equal concept in the hierarchy (sibling) or a concept itself. When you roll up, the highest parent ( the most broad term) will have the largest level of separation. And yes, there can be multiple parents/children/siblings at each level. Eventually it sometimes rolls up to one single parent (like 4008453 SNOMED CT concept).
A combo drug with multiple ingredients is another example of multiple parents: each ingredient will be a parent of the combo drug.