The 20-device limit applies to both routers and end devices. If you have (almost) 20 routers that are directly connected to Homey, you will hit the limit as well.
What may work is to add (end) devices at, or at least close to, their final location. That way, the device will hopefully join the network through a router. Some devices don’t “jump” routers and will prefer to use the router they initially used to join the network, which may not be the most logical router in the mesh.
Also, it may work to completely turn off a few of your lights when adding new devices, hopefully forcing those lights to pick a router when they get powered up again.
And also take note of what @RoyWissenburg says: having lots of routers close to each other doesn’t necessarily make good coverage for your entire property.
