If you want to learn about batteries and their characteristics, you should probably go to the BatteryUniversity to learn a few things before you start to do calculations, because if you miss interpret stated parameters as facts, you will get wrong results.
https://batteryuniversity.com/learn/
The specific mAh you can get from a battery depends on the voltage difference from charged to discharged state you are willing to use and at what current you do it (carefullcareful it can be dangerous or damaging to discharge to low or charge to high as well as overdrawing current) and what you. The way to be sure is to measure when you charge or discharge. Stated capacities on batteries are estimnates at best, depending on battery type it can be the most achivable mAh or the minimum value which is always achived ... check your datasheet of the battery for this(if your battery has no datasheet then good luck finding out bythe only option is measuring).
There is some websites and youtube channels doing battery tests, only if you test a cell you can know exactly how it performs.
LIION:
https://lygte-info.dk/review/batteries2012/Common18650Summary%20UK.html http://www.dampfakkus.de/akku_liste-nach-groesse.php?size=18650
LIPO:
very difficult to find tests because the batches of batteries even from the same vendor have different performance characteristics. Find some here:
https://rightbattery.com/lipo-liion-life-batteries/
NiMH/NiCd
https://rightbattery.com/nicd-nimh-batteries/
theres are more resources but thats just what i found in a moderate time frameI hope this gives you the right pointers on where to start.