Interesting. I couldn't find anything in any of the documentation, either...
The P212S says up to 12 amp hr.
The P624 says up to 10 Amp Hr.
The P983 says up to 2 8 Amp Hr Batteries.(which is really a P624 for a cellular uplink...
I would stick with one battery just in case. Until someone from Elk says differently, that is.