hope the following quote from Situ Rinpoche's book 'Ground Path and Fruition' will be of help.

‘Some very old people complete 5, 6, 7, 8 or 10 hundred million recitations of
Om Mani Peme Hung. We call a hundred thousand recitations a ‘tung’. This literally means a conch shell. They also use prayer wheels. Prayer wheels are spun clockwise, a conch shell is used underneath the prayers in the wheel and there is bamboo between the two. Eventually the conch shell breaks the bamboo and that is supposed to equal one hundred million Om Mani Peme Hung recitations. I know people who can complete a conch shell in a week in this way. Normally it takes six months, but if you are a really good practitioner and use it all the time you can do it in a week. Of course they are not using iron, they are using bamboo. If they were to use iron they would finish a conch shell in two hours, but not with bamboo. Some of these people then collect the finished conch shells to use as a mala. Malas like these show they have finished all these conch shells.’