These days DD Boost integration with Data Domain and Networker does not require a Data Domain appliance at the remote site. In fact, DD boost Virtual Appliance can be hosted and can do the functionality of the former. This would reduce the cost of remote sites drastically. Require EMC technical panel to validate the same and also on the licensing part of it.
DD VA is not generally available and as such (from what I have seen at least) is limited in number of supported streams. Of course, it might be perfect fit for you, but right now this is not generally available. I'm quite sure that soon or later it will become available as there is "softwarization" trend everywhere for anything HW related where brain can exported and used with commodity hardware.
Streams are a model specific value - it would depend on which model you had.
Regarding the number of streams supported for each DataDomain model, please check: