Maximum Secondary DNS server in a network?

In a large enterprise network of w2k3 domain, I understand there should be only one primary DNS server.
Our Domain is a geographically disparsed one with branches in a many cities, the Primary DNS is the HQ,
most of the branches needs to have secondary DNS server, the question then is; what is the maximum DNS server to be deployed in this entire network? any limit to this - regarding w2k3 domain?
this are all Active Directory integrated DNS servers.

Two secondary servers are the maximum but that's not to say you couldn't have several more on standby which you could activate manually.