Malazan Empire: A question about the wraiths in MT - Malazan Empire

Ok, I'm a bit confused here. Whither tells Udinaas that he is not bound because he was not one of the Tiste Andii slain at the hands of the Edur. Thus, he is not bound. (I don't have the book with me so it could actually be the female Tiste that tells this to Seren Pedac in Kurald Galain when she is with the Crimson Gaurd).

So, according to the observations of Scabandari, less than a thousand Tiste Andii remained at the end of the invasion. Thus, why are there so many wraiths in the Tiste Edur army when they march on Letheras? In one scene they are described as been 'beyond counting'.

This doesn't make much sense to me. Anyone have an explanation?