I'd go with:
The number of unit commands given, not counting commands that are duplicates of the previous command (with the same unit selection) within the same 1s.
Eg. pressing stop with unit selected is 1 command
pressing stop 2x in 1s with same unit selected is 1 command
pressing stop with one unit selected, then pressing stop with a different unit selected, within a 1s window, is 2 commands.
Clicks and key presses that do not issue a command are not actions. Giving a command that is almost identical to an existing command to the same unit selection in the same short timeframe (eg. a move command to a location within 5 elmo of existing move command) are not actions.
That way you only get meaningful readout. The stuff starcraft players do to boost APM like rapidly clicking a rock are sad.