You need to work out or measure the current draw of your motor at full throttle and max pitch. This will determine the size of your esc. If you connect the batteries in series you will have 5400mah at 22,2v still at 20C The "C" rate is the maximum available current draw of the battery and is measured in mutiples of battery capacity, for example a 20C 5400 mah battery would give you 108A maximum, as stated by BarracudaHockey. The C rate of the battery is determined internally by the construction of the battery and you cannot change it by connecting it in series or parralell. If you want a longer flight time you will need larger capacity batteries with more cells i.e 2x6s 5400mah batteries. You need 6s batteries to be able to sustain the 22.2V that the esc needs.