X-10 has no way to handle more than 256 addresses. ACT offers a variant they call A10 which allows for more addresses. A Google search on "ACT A10 PLC" should turn up many links.

Useful distance will vary. The powerlines act like a network, the impedance of which, affects signal strength. There are several papers on this topic at...

[Link: info.iet.unipi.it]

Another URL helpful on this topic is...

[Link: tenet.res.in]

X-10 transmitters load the network so the more transmitters used, the lower the signal strength.

SmartHome has introduced some devices (BoosterLinc) which can boost signals in realtime. They are still rather new. Past experience is that any product from SmartHome needs a few years to mature. Their devices are also only for 120V systems.

The utility company's distribution transformers tend to block the X-10 120kHz bursts. In N. America, the typical transformer serves 4-6 residences. In Europe, many residences may share a transformer.