How important is it for a child to have a limited number of caregivers?

Question

In the context of instituational child care, I have heard that a child needs to form an attachment with a limited number of people to function properly. However, now when I tried to find some sources for this claim, I have only found studies which say

1. It may be connected to the simple time and energy a caregiver has per child (Schipper, 2006; Palmerus, 1991)
2. Some evidence suggests quality institutional daycare of preschool children doesn't disrupt their parental attachment.
3. Polyamorous families don't seem to be a disadvantage in raising healthy children

So is the number of important caregivers something important?