I think that once a primary bond is established to a caregiver, then more attachments can form to more people. my daughter has strong bonds with me primarily... i have the boobs and do most of the taking care of her, then my husband and dad, she then has more with the inlaws and so on. once the primary bond is found they can go on to have more with others.
bonding is absolutely critical for the best emotional development... you can see that in cases where children are damaged by not being loved, or have issues of not being attached.
if a good attachment is not formed in the critical period (birth- 2.5 years i think?!) or an attachment is broken (ie mother goes to hospital as she did with my dad, his aunt took care of him to which he then bonded, and then his mother returned, and his aunt left) then that child will have issues bonding/making attachments with people later on.
parents bond differently with their children, i don't know about things being 'equal' or if its even possible to measure... because they are very different.
(its been a while since i studied this stuff, i hope i'm not talking crap here)