I'm currently proposing the PnPSCADA pre-paid option to clients, using the Elster AS230 meter. How many PnP channels are required for this as a minimum? Of course, I imagine that if we want to also do advanced energy management, additional channels need to be catered for, depending on what can be read from the meter. Please advise. Thanks!

You'd typically need one channel for reading the kWh, and a second channel to control the Contactor, for a total of 2 channels per house.

Please note that the AS230 can also read reactive energy, so it could be that you're also reading that. To stop reading that, go to Edit->Advanced Meter Settings, and block it (under 'Meter Channels to Ignore').
If you do not block it and you read reactive energy, the meter will use 2 channels for reading Energy, and a third channel to control the Contactor, for a total of 3 channels per house.