WiFi as a protocol doesn’t use repeaters The way the mesh protocols like Zigbee, Z wave, and thread do, so you can’t increase the max by adding a repeater.
The number of available Wi-Fi slots is determined by two things: the limit of your specific Wi-Fi router model, and any limit that your Internet service provider may apply.
If your router has a hard maximum of 28, 28 is all you get. You would have to change to a different model of Wi-Fi router to get more. It’s one of the reasons that Zigbee and Zwave became so popular for lighting: they don’t require Wi-Fi slots.
Matter doesn’t change anything with regard to the number of Wi-Fi devices you can connect.