I am travelling from Riga to Stockholm with AirBaltic flight carrier. I only need my laptops: 11" Chromebook, 15" Lenovo Thinkpad, and 13" Asus Zenbook. The weight is not too much. They all can fit my backpack.
There are three batteries: two integrated (Chromebook and Asus), and one which can be deattached (Lenovo). I did not find any facts about the maximum amount of batteries allowed.
I also would like to take one iPad with me, one Pebble smart watch and one phone (Oneplus 2). I think the situation should be ok. So 6 batteries altogether, but 3 in laptops.