Firstly that’s a lot of devices and secondly if you actually have that many IP addresses on a network, your issue is not a home assistant specific issue. Its basically a networking issue. The solution to which is to setup multiple networks as you mentioned, and simply setup routing between them.
Configuring routing between networks is a router specific task, and no doubt there are a lot of tutorials available on the internet to help with this.
I wouldn’t worry too much about it, most the problems you’d face with a lot of devices is broadcast traffic and network efficiency. Whilst I haven’t specifically checked out the broadcast traffic IoT devices, they’re very unlikely to be as chatty as say a few hundred windows devices.
You’re far more likely to hit issues with wireless performance and max devices that the particular radio your AP supports.
Personally I have a separate network setup for my IoT devices, with no route to the internet, on their own SSID. But I wouldn’t get too hung up with it to begin with, you can expand your network as you get more comfortable with the setup down the track.
Not only that, if you reach a point where you have that many devices I would start thinking about the power consumption of them. They use next to nothing individually, but if you’ve got 400 of them, combined it’d be like running an old incandescent light globe all the time.
Not all smart devices are connected to your home LAN network - a lot of sensors, smart plugs, bulbs and other devices are using different protocols like zigbee, zwave, etc.
For example one hue bridge support 50 LED bulbs, that’s just 1 IP address for the hub from your IP range.
Even if you hit more than 256 devices connected to your home LAN network - what stops you from using a bigger range? e.g. 192.168.1.1 to 192.168.2.254 - it is still a single subnet. Or you can have multiple subnets, depends on your network equipment.
I would say you can connect unlimited number of devices to your system.