Board Thread:Questions and Answers/@comment-24826159-20140416214618/@comment-957629-20140417002431

We don't know how Vasto Lorde were created so that's the only thing shrouded in mystery!! However, the other Menos are an amalgamation of many Hollow souls creating a bigger more powerful creature!! Even Adjuchas were this but they ended up with a more dominant Hollow that suppressed the rest!! Hollows came from souls that were lonely and tied to the Human World and eventually lost their hearts that turned into masks!! So in a sense then yes each Arrancar is many hollows or a single hollow and thus many souls!!