If you don't use too large numbers you can count rhythmically.
Try to think of four times
MAUM MAUM - MAUM MAUM
You want to get to 18 repetitions
[MAUM MAUM - MAUM MAUM]
[MAUM MAUM - MAUM MAUM]
This is eight repetitions. Do it again.
So it becomes 16 repetitions because you did it twice. To get to 18 you have to vibrate MAUM two more times. Do that and you are there.
If you don't use too large numbers you can count rhythmically.
Try to think of four times
MAUM MAUM - MAUM MAUM
You want to get to 18 repetitions
[MAUM MAUM - MAUM MAUM]
[MAUM MAUM - MAUM MAUM]
This is eight repetitions. Do it again.
So it becomes 16 repetitions because you did it twice. To get to 18 you have to vibrate MAUM two more times. Do that and you are there.
A method becomes simple with time. Nothing is complex and nothing is simple. It is only we who become more expert in a method and this is achieved by using the method. Those who are developing some siddhis know this well. As you use that method you will see that this continuous grouping system will become more and more orderly and you will be able to manage it better and better.