what kind of amperage does this heater draw?
Your device has 3 heater elements - so for the 9kW version, each element produces 3kW of heat.  

Your drawing indicates that each element is fed with a 230V supply - so 3kW from 230V requires 13A each.  

Now if you refer to the "Circuit breaker" column you'll see that your 9kW device can be supplied by a 3-phase supply with 3 x 16A breakers, or if you connect all 3 elements together to the same single-phase breaker then it must be 50A.  

The "Power cable" column appears to be telling you what size cables you need, and the numbers given include the Neutral and Earth/Ground wires.
So if you feed your 9kW device from a 3-phase supply, you need 5-wire cable, each wire 2.5mm2, or for a single-phase supply you need 3-wire cable, each wire 10mm2.