e-cigarette?

I think you somehow got something wrong… there’s no “Here you have to take XXXX mAh batteries.” mAh is the indication WIEVIEL current fits into such a battery, comparable for example with the milliliters in bottles or the (Kilo)Gramm in food. For the individual e-cigarettes, however, it is only important how much current can be taken from AUF EINMAL without overloading the battery. This is called “maximum discharge current” and is indicated in Ampere (A) and the recommendation of many battery carrier manufacturers is upwards from 25A, as Franky already wrote. If you do not find any information in the description, then you will be safer at 18650 batteries with the Murata Konion VTC5A, VTC5 or VTC5D… they have all 30+A discharge current… a suitable link to a cheap and trusted shop has also been provided to Franky. If you need 21700 cells, these are among the most recent: https://www.nkon.nl/de/rechargeable/li-ion/21700-20700-size/samsung-inr21700-40t-4000mah-30a.html