30 amps, depends on what his mains supply rating. He's got 3 phase for his Ac , he may have upwards of 50 amps per phase depending on the capacity of the mains cable or more.Plenty of juice for 2 x 15 amp separate circuits for 2 x 3600w elements.
I wouldn't run seperate 3 phase circuit and then tap off seperate single phase elements as you would need to provide one large 3 phase cable to carry the loads and provide an adequate neutral return. Then you'll need to muck around with too much wiring crap,sub board etc etc,I'd just run seperate 20 amp 2.5mm cables ( depending)with 15 amp points with rcd protection at main switchboard . Run 3, 1 off each phase and balance your house load at main switch board.
Even with the air and stove/oven and other stuff in the house sounds like you got heaps of juice unless someone's just run 6 mm three phase to your main switch board. Also will depend on factors like mains cable run lengths other installation deratings etc, you are defiantly needing a domestic sparky with max demand experience in designs when stepping into this territory now.
. I'd be getting your sparky bloke to come around, do a balanced load over 3 phases, and if 10mm or bigger mains 16 or 25 would be awesome , tap in 2 -3 seperate 15 amp circuits with 20 amp rcd protected breakers. **** load of juice for your setup, you could then just use your power points for pumps etc. this is proballly over doing it, but it's what I'd be doing if you are wanting to run 2 x 15 amp elements plus other things, possibly an 10 amp urns etc
Without seperate circuits, you will be limited to a couple of 2400 watt elements running from 2 power circuits. 10 amps on each 20 amp power circuit would be no issue unless you start plugging in kettles, wife's hair drier etc, then you'll trip the breaker.
Those combo breakers you got there are good ones , but going bigger elements, get your sparky friend to come and check your consumer supply and load ratings.
Most large modern domestic 3 phase Ac systems (20-25 kW) draw approx 9-13 amps per phase on full load, so if you got 50 amps per phase or more then you'll piss in a couple of full load 15 amp circuits as well.
As long as you get your phases balanced out .
btw, 2 x 2400 elements would be plenty to get your kettle boiling anyway, if you got 2 power circuits then you'll be fine, looks like you may have 3-4 so even better, you could run an urn and if wanting to ramp the boil up quicker do what I did and throw an over the side 10 amp immersion heater in for 15 mins, that'll ramp it up fast. Got mine for 80 bucks, saves lots of time.
For more juice, wire it up with dedicated circuits