Regulations limit the total battery energy you can carry on board, which would be measured in Wh. Usually the limit is 100Wh though some countries/airlines have different regulations for total vs individual capacities (e.g. max 200Wh total but each device cannot be over 100Wh).

For regular Lithium-Ion cells which are usually 3.6 to 3.7V, 100Wh is around 27 000 mAh. Always check the battery cell voltage though, since it's pretty easy to claim any mAh the company wants since it's not really a measurement of anything tangible.