Depends. If it’s a generally loving household, the more the better.
If it’s selfish love, then the kid sees examples of jealousy and fighting amplified by the number of partners.
The problem with the cultish polgamous families is when they’re abusive. But that’s a problem with any family structure.
Perhaps the benefit of paired marriage is that it’s simpler: Debating is between two people and not a quorum and of course in a really good working family, the child is included in decision making.