Number of wireless units

You will reach a practical limit before reaching a physical limit.  Your practical limit depends on a variety of things:
- Single or dual band router
- Processing power of the cpu in your router.
- Speed of your shared internet connection.
- What each device is doing.

Obviously, streaming 4K video on 5 devices requires more power and bandwidth than checking e-mail on 20 devices.  Sorry I can't be more specific.  The answer isn't a simple one, depending on the power of your router and speed of your connection.

That being said, are you talking about 10 to 20 devices or 100 to 200 devices or more?  What router do you have and what is the speed of your internet connection?

Wouldn't the limit be the number of IP addresses the DHCP server (modem or router) allows? 

Not necessarily, Jerry.  A class C address (like 192.168.0.xxx) allows 254 addresses.  Netgear's KB says "NETGEAR home routers can accommodate up to 32 clients per wireless band. If your router is dual band, the total wireless clients your router can handle is 64 (32 for the 2.4GHz and 32 for the 5GHz)."

That is a physical limit, but 64 simultaneous devices may not be practical.  The KB goes on to state: "However, since the router's wireless channel is shared between all the wireless clients, adding clients will inevitably result in slower network access for all clients. This will be particularly noticeable if some of the clients are using a lot of wireless bandwidth, for example by watching a video or doing a torrent download. Therefore, the maximum number of wireless clients that will operate satisfactorily while connected to the same router will vary depending on what the devices are used for. It will also vary depending on how much wireless congestion or interference are present in the location where the router is installed."

This is like WiFi speeds.  Wireless N300 theoretically has a limit of 300Mbps.  In practice, you may get 70-80Mbps but often you are limited to 40-50Mbps.  Number of WiFi devices is no different, what is practical no why approaches the theoretical limit.

If you do streaming and have Cox 50Mbps service:
- 1 TV streaming Netflix in 4K = 25Mbps
- 4 TV's streaming Netflix in 1080p 5Mbps each = 20Mbps
That will saturate both a wireless N band and your broadband connection, so, the limit in this case is 5 (and maybe that's too much for acceptable response).  On the other hand, the same router may be able to handle 20 to 30 or more devices just checking e-mail.