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)
In my experience, there is no limit to the number of circles that you can have because personally have 177 circles as when I started using Google+ I wanted to put people in as many categories as I could think of where I might want to group people. For a business, this could be people who've bought your sweatshirts and those who bought Izod and those who bought books, etc. Obviously one customer could fit in all of those categories. One person I have in 31 different circles.
As to how many people you can have in a circle, I think that is limited by the 5000 (individual people and business pages) you can follow as I have one circle with 618 people in it. But if you are creating "shared circles" the limit is 500 people. Likewise, Google+ will limit any messages to a shared circle to 500 people.