IATA says a limit of 2 <160Wh batteries (with airline approval) and a limit of 20 <100Wh batteries (which is a lot!), but also airlines are free to be more or less strict as they see fit, so basically check with the airline (it's rare that they'll be less strict as a policy, but they may be OK if you clear it with them beforehand, and some are more restrictive by default).