Up front, to my knowledge the R6220 does not support Netgear Armour (BitDefender) - this is where you filed the question. Better suited would be the General WiFi Routers (Non-Nighthawk) section.
Haven't located a max number - but what would you assume from such a very basic and low cost device? Ideally one might assume it's good for a complete /24 subnet, so some 250 MAC addresses. Unfortunately, Netgear is a little bit lazy not publishing such specs. Let's hope somebody from Netgear will be able to answer.
