CSM FAQ (Plague Zombies, etc)

http://www.games-workshop.com/MEDIA_CustomProductCatalog/m2710181a_Chaos_Space_Marines_v1.0a.pdf

Page 61 – Typhus, Plague Zombies
Change to ‘Plague Zombies are Chaos Cultists that have the
Fearless, Feel No Pain and Slow and Purposeful special rules
and cannot purchase options other than to add additional
models to the unit.’

So no, you're not limited to a unit of 10. And yes, I am already planning a shambling horde to accompany my new Death Guard army.

Yeah, the argument you could only take 10 was another classic "internetz lul" argument.