The Utility Network does not model cardinality, but it can be enforced, restricted or flaged as error with an Attribute Rule. Here is an example of a constraint rule I created to reject too many related features by type. This could also be done as a validation rule where instead of preventing the edit, it would just mark the issue as an error when the rule was violated.
https://github.com/MikeMillerGIS/arcade-expressions/blob/master/attribute_rule_constraint/reject_too_many_related.md