Do I need to count the balls in each over?

Yes, keep your own count so you can back up the main umpire. In Stage 1, wides and no-balls aren't re-bowled, so every ball bowled counts towards the six.

When there are two balls remaining in the over, the main umpire (at the bowler's end) will check with you, usually by holding up two fingers. Confirm by giving the same signal back if your count agrees. If your counts differ, sort it out with the main umpire before the next ball is bowled.