If you look at the “Use in North America” page in the ‘Learn’ section, you will find a large range of C.T’s that can be used with the emonTx and emonPi. Some require a small modification to the hardware.
Strictly speaking only one. That is because only one radio channel is available, therefore only one emonTx can transmit at a time. Therefore, the practical answer is: it depends on how often the emonTx’s transmit to send the readings. The fixed limit for the total number of “Nodes” on the radio channel is 29. You are unlikely to experience satisfactory operation with 29 transmitting at 10 s intervals. The realistic number is probably around 6.