My brother is my best friend. We do not live with our mother, and our father is hardly an appropriate attatchment figure. I do not have friends i socialise with besides him, which often results in me being left out. He is growing up, and wants to do his own things without me. Since i have no-one else to look to besides connections online, it is causing a lot of upset.
Therefore i think Humans DO need multiple attatchments in order to me mentally healthy. But how many, and in what form i think depends entirely upon the individual.
If you're happy, i don't think anyone can argue with that.
If you ask yourself truthfully, and find out you're not, then new attatchments might be just what you need.