Questions about mandrakes.

I know that mandrakes are supposed to be useful this edition, but my question is how many would be worth running in a game. A small five man? 2 ten man?

Also, I glanced around but maybe someone knows off the top of their head... Does a -1 to hit (shrouded from sight) mean that they cannot be hit by overwatch?

[Shrouded from sight] does not make them immune to overwatch because modifiers do not apply to overwatch.

At a Doubles tournament a few months ago, I ran three units of ten (with some Scourge, and allied to Wraithguard).

The only reason we didn't come 2nd/3rd was because of naught shenanigans on the part of the team we were playing.
Those Mandrakes did VERY well for themselves.

If you're gonna take Mandrakes, take at least 10. They're good at killing infantry in cover, good screening if you need it, good for chipping mortal wounds off something with an invuln (demon prince), all around a super good unit. They're a good counter unit as well, since they're good against things that are going to get in your face like demon princes, Blood Angels, that kind of thing. Wait for them to come at you, then drop and shoot.

Overall super versatile. Expensive though, but they fit well into a Venom spam list if you ask me.

Thanks for the advice everyone. I think I will pick up 10 and give them a try.

I have 20 (ended up selling third unit because it was finecast and one look at those toes dissuaded me from even trying), and they are amazing.