The only hard limit I can think of is that a Couchbase Server doc can’t exceed 20MB, and SG stores a list of a document’s channels in the document as invisible metadata. So a few million channels would start to overflow this.
There are likely to be performance problems with huge numbers of channels on a doc, though, but they’re hard to characterize because they depend on the exact usage scenario.
It would probably be better to define a channel for the document, then give the subscribed users access to that channel.
—Jens