In term of recording planks, 1 plank is 1 set, and record the number of seconds you held it in the reps column.
Probably start with holding a plank for 30 seconds. If you can do this comfortably, then hold it for longer next time.
I'm not sure I have seen anything firm on this, but once you can hold a plank for 2 minutes, it's probably time to move up to a more challenging version of the plank. From modified plank to plank, or from plank to single leg plank.