If it's not already clear to you, let me add this. There are three sets of pistons, each set are at TDC at the same time, 120 degrees apart First set ( 1&2) are first then 120 degrees later (4&5) then finally 120 degrees later (5&6). The first round, #1, 4 & 5 fire then the second round # 2, 3, & 6 fire.
Thus the firing order is 1, 4, 5, 2, 3, 6 . It takes two rounds to fire all six cylinders.
To answer your question, the three crankshaft pairs are 120 degrees apart and a power stroke occurs every 120 degrees, three per each crankshaft turn.
Cracker
