There is no physical limit to the number of accounts that a client can implement for their Unified Messaging Channels.  The main limitations would be in creating and managing the individual Facebook pages and Twilio accounts.  It would be interesting to learn more about the use case that would want to engage on that many channels!  In general, it would be best to check in with the product team if the use case was planning to engage on more than ten channels at a time.  Aside from Facebook and Twilio, because of the Twitter API Limitations, clients should not configure more than 5 Twitter channels on their instance without use-case discussion.