You can only work iCloud at this time with a single Apple ID, and therefore a limit of 10 devices per ID.
Found this on the web
Full article
http://www.tuaw.com/2011/08/09/apple-id-account-device-limits-gain-attention/
Short quote
When you use Apple's new multi-device download feature, you will be limited to a total of 10 devices and computers, each authorized with the same Apple ID. What's more, once a device or computer has been assigned to an Apple ID, it cannot be reassigned for 90 days. Apple's Knowledge Base support article details how you can deauthorize devices, and how to check the wait time before they can be re-assigned.
From apple support
http://support.apple.com/kb/HT4627
When you turn on Automatic Downloads or download past purchases on an iOS device or computer, your device or computer will be associated with your Apple ID. Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days. You can view which devices or computers are currently associated, remove unused devices or computers, and see how long before they can be associated with a different Apple ID from the Account Information page in iTunes on your computer:
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