I think the real question specify what kind of self-confidence/self-esteem is involved - the kind that a kid gets from, saying, vying for attention or stuff and generally fighting his way, or winning (maybe even bullying or else beguiling) his way through childhood? the kind that seems automatically bestowed (no matter who/what the child is) by either parents or other adults or older siblings? OR, conversely, the kind that may be at risk by one or more parents, other adults in a child's a life and/or siblings?
It just depends on the child and the family and how well the child has been shielded from the things, behaviors and attitudes that tend to take away from a child's sense of self-esteem and/or self-confidence.
Then again, the question might be weather it's a healthy self-esteem/self-confidence or whether it's over-inflated and/or out of balance in some, or all ways, ways (as measured by objective standards in the mental health field and within the context of healthy relationships of all kinds), as well as within the context of functioning in the world outside just the childhood family.
Kids need the right, and objective, kind of perspective presented to them (so they need parents who can step outside themselves and approach their child, parenting, and family not just from within that narrow perspective, but also (and at the same time sometimes) from a "larger picture" (outside world/standards) perspective.
I don't think how many kids there are matter if the parent(s) knows how to manage the number of them well; and if the parent is not all that great at making sure every child has what he, as an individual needs, and what he, as an individual, shouldn't have to put up with; well, then, sometimes some children are better off not having a parent zero in on them too much.
I think spacing is a bigger factor (for good and/or ill) than many realize - not necessarily number of children and maybe not even necessarily level of family income.
Too much of the wrong kind of self-confidence (or, as far as I can guess, self-esteem, for that matter) is not particularly all that great a thing (for the individual, himself, or for others who deal with him or his thinking).