The only thing I can find is here:
http://inaic.com/index.php?p=registry-operations#8
which states the limit is thirteen due to UDP limitations.  
But that isn't a "legit" company.  
I looked in FRFC1035 and some others and can't find this.  I also have a Bind book and looked at all the NS records listed in the index.  Nothing there.
Thanks
Paul

Seriously? no one?

The only thing I can say regarding a limit is that based upon practical experience I have seen problems associated with oversize DNS (UDP) packets.  For example, Qmail has a problem with larger DNS entries as it chomps the packet at 512 bytes and may miss critical information, such as the MX record.
 

In my experience, there isn't a 'hard' limit as such (ie X RRs); it is based entirely on the SOA packet size. If you bust the 512 bit UDP limit your BIND DNS server with either start to truncate (drop out the A glue records) or possibly switch to TCP (depending on version and configuration).

To keep the packet size as small as possible, have your NS records in the same domain (eg ns1.mydomain.com, ns2.mydomain.com etc and not ns1.mydomain.com and ns2.someotherdomain.com) as a flag is used to identify the domain where the same is used multiple times. Also, the shorter your domain name and NS names, the smaller the packet size.

Hope this helps.