Hello Mikkel and Richard,
This is what I know, some of it through bitter experience
with the Envi and the EnviR there are 10 "slots" in which you can tune transmitters to the monitor. These are sometimes called appliances (appliance0 -> appliance9).
If the transmitter is like this
you can plug into it 3 sensors (Current Clamps). Other transmitters may allow fewer sensors [IAMs - 1, Optismart - 1 (and may take up 2 transmitter slots)].
On the monitor the values from the 3 sensors are added together before display so that you see a single value for each of the appliances. [I assume this is because the original intention of the 3 sensors was to measure 3-phase supply].
If you want to see the values from individual sensors you must connect your Envi(R) to a PC using the current cost supplied cable. This gives you access to XML formatted messages from the transmitters (one message from each transmitter every 6 seconds). The values from each sensor are buried in the message.
So in that sense, it is possible to get 30 readings per Envi(R).
The above is mostly theoretical. In practice as you add more transmitters the risk of collisions between messages increases and the Envi(r) appears to drop collisions. [With 4 transmitters I sometime see a run of 60 seconds without a message from one of the transmitters].
There is no commercially available software to process this data, though a number of user supplied packages are available which make it easy to get at the data. Alternatively, with moderate programming skills you can process the messages to obtain the information you need.
Hope that Helped.