I figured useable capacity would be anyone's guess, so I didn't want to ask such an open ended question! I'm thinking 10-12 devices would be actively in use at a peak time, mostly tablets or mobile devices, web browsing/social media, video sharing websites, and mobile gaming. Located pretty much within 30ft of a base station.
The more I think about it, the more Q0S by device would really be useful so an adult with a laptop doing work gets priority vs the 10 kiddies with kindle fires not doing anything productive. I guess I could hold my breath waiting for Netgear to release such a feature in an Orbi firmware update, or get the right product now in a "BHR".