The transmit power is the JeeLib default of +7 dBm, though by modifying the code it can be turned up - but clearly at the expense of battery life, as the radio is by far the biggest consumer of power, even though it only transmits for a very short time (less than 10 ms) every minute.
Each device on the radio network must have a unique NodeID, those are limited to 30. So that’s the absolute maximum. But you must also consider that each emonTH won’t send its messages at exactly the same rate (due to timing variations) so you will get “collisions” when two messages interfere with and destroy each other. That’s unpredictable, so the real answer is, how many lost readings can you tolerate?
The currents taken from the data sheet of the RFM69CW:
+13 dBm, 45 mA
+10 dBm, 33 mA
0 dBm, 22 mA
-1 dBm, 16 mA
Sleep mode: 1 µA max.