Hopefully this question is for home use rather than work. Otherwise you'll probably want something other than a wireless router (more like an access point as outlined above).
In theory, there shouldn't be a limit to the number of wireless devices that can use an access point. There is a finite amount of bandwidth, but as long as everything isn't hitting the router simultaneously that is less of an issue.
In practice, you are typically limited to 254 devices. But this is usually only because the router is passing out IP addresses via DHCP and only allocates a /24 subnet (256 addresses, one reserved for the router and one for the broadcast address). If you had a router that could handle DHCP scopes for larger subnets you could, in theory, do more. Keep in mind that this address pool is also shared with the wired devices, so if you hang wired PCs off of that router (or off of another switch behind the router) then the pool would be reduced.
The only other place where you might run into a limitation would be if you secure the router via MAC address. Most wireless routers allow you to restrict wireless access to only specified MAC addresses, and depending on the router model there may be a finite number of MAC addresses that you could specify.