DISCLAIMER: This is all from memory mind you, and I wasn't the one playing Sivir, my sister was, and I was supporting her. So I wasn't exactly looking at her mana 24/7...
However, the logic that follows is:
1.Sivir w - if Sivir critically strikes her target, the bounces do so as well
2.
- UNIQUE passive: Critical strikes restore 3% of your maximum mana pool.
So if Sivir crits on 6 separate enemies, she should get 18% mana back (following those two facts).
But I am going to test this to make sure.
Edit: tested it, and it does not. I would think that the mana restoration is a on-hit effect that procs when the attack proc's, and the bounces of Sivir's do not proc on hit effects. That's how it makes sense to me at least. Maybe it did work like that at one point, or maybe it was an idea created by the slight vagueness of the wording. I really don't know.