The Unifi HD access points says that it can run "500+ concurrent clients" for one access point: https://store.ubnt.com/products/unifi-hd
What does it mean by this? I understand that it's out of the question to run 500 high usage clients, but what about low usage clients?
As a related question, what's the theoretical highest number of low-data concurrent clients one access point can run?
What does it mean by this?
It means it can have 500+ clients connected at the same time. It makes no claims about the performance with 500+ clients, but the hardware/software of the AP itself can handle 500+ clients.
Many APs (especially consumer or small business devices) don't have the capability of handling a large number of clients. They tend to fail in spectacular ways when they are exposed to large client loads.
I understand that it's out of the question to run 500 high usage clients, but what about low usage clients?
Again, they make no claims on performance, so high usage or low usage really doesn't matter to their claim.
Perhaps their claim is based on sending a multicast stream at 100Mbps to 500+ clients in a controlled environment or it may be 10kbps to 500+ clients. The AP still supports 500+ clients in either case (or any other).
As a related question, what's the theoretical highest number of low-data concurrent clients one access point can run?
Unanswerable. This would be specific to the hardware, software, configuration, and/or environment in question. Some devices start having issues with as few as 30 clients. Others can have hundreds connected without issue.
For example, there will be a significant difference in the (even theoretical) answer between the linked Ubiquiti product (two radios) and a Cisco 4800 AP (four radios).
