I think your going to need a Balance One with Speedfusion License or the 210, 380 and up models if you want to actually bond your ADSL WAN lines. Last time I checked the B50 didn’t have Speedfusion, I could be wrong with their later firmware releases maybe there is now an option to upgrade but one of the team guys would have to jump in on that one.
If you have that many devices in each of your LAN then I don’t think the B50 is what your looking for, your probably going to want to look at the B710 or above. You need a lot more power for subnets that large unless you have it that way for a certain reason.
If you have the 4 WAN and below and no more than 100 devices on the network you can probably squeeze by with a Balance One with Speedfusion upgrade IMO. 100-400 range I play it safe with the B580 or even B710 depending on the traffic you push through it and VPN requirements. At the end of the day to give you a more accurate proposal you would need to provide much more detail.