The short answer is "maybe 25" if you allow both bands to be used simultaneously.
(which is not the use case you described)
Keep reading if you want more detail.
I don't know what the "official" answer to this question is but since you're looking for a theoretical (pie in the sky) number:
I'd look at the info Ruckus provides for the AP:http://www.ruckuswireless.com/products/access-points/zoneflex-indoor/zoneflex-r500
...ignore the 5Ghz max theoretical bandwidth (since you're specifying 20Mhz channelization) and substitute the 2.4Ghz figure of 300Mbps, divide by the 5mbps you specified, and arrive at a figure of 60 clients.
I would never expect to get close to "theoretical and optimal" performance, but since I'd configure my APs to support 40Mhz channelization (or more w/ ac support) and I'd expect some number of clients to support higher channelization and netflix should provide enough caching to smooth over momentary congestion issues...
Well... You're still not going to get close to that theoretical number.
WiFi might not be your only bottleneck.
Do you have 300Mbps of internet connectivity?
Here are independent throughput results found on the Ruckus website:http://www.ruckuswireless.com/carnet-performance-testing
If I'm reading it right, the effective throughput of an R500 (with 60 clients pulling traffic) drops to 80Mbps, leaving something better than 1Mbps per client.
If we use 80Mbps as our bandwidth cap, you get 16 netflix clients. (at 5Mbps each)
...but the "real world" nature of those results were disputed by other vendors.
Tom's Hardware claimed that a Ruckus ZF7363 delivered 111Mbps to 60 clients.http://www.ruckuswireless.com/press/releases/20110718-independent-test-reveals-ruckus-outperforms-ot...
A more relevant test was conducted against a ZF7982:http://www.ruckuswireless.com/press/releases/20130219-ruckus-smart-wi-fi-takes-top-marks-in-non-vend...
Their conclusion was that 25 iPad clients could simultaneously stream "HD" video through a single 7982.
So, compare the advertized performance numbers of a 7982 to those of the R500, scale the results appropriately and... you might get 10 or 12 simultaneous video clients per band.
I'm guessing, 'cause I didn't do the math there.
Plus, we totally lost track of your 20Mhz channelization specification.
The tests that Ruckus is highlighting seem to indicate that other brands would do worse.
..but feel free to search for your own throughput stress tests or conduct your own.