I can't find in the Google+ help anything about a limit on the number of circles that brand pages can create. One of the best things about G+ for brands is the ability to segment and target people according to whatever criteria you want. But is there a limit on how many circles a single brand page can create?
I assume it is the same as a Person's page, you can follow 5000 entities. The equivalent in circles I am not sure, but as long as you put 5000 users within your circles, that is the limit. The number of users that can follow you seems close to unlimited though.
(Source: Scoble - https://plus.google.com/111091089527727420853/posts hit it in September, the actual post where he says that, is too hard to find now)