It honestly depends on a few factors. Mostly… what games are you planning to play? There’s stuff that can last for 5-8 hours with the Deck battery alone. Here’s a good bit of information: mentions both a maximum of 100wh on power banks and how to calculate wh from volts and ah (V * Ah). So assuming the usual 3.6v power bank the maximum you can carry is a 27000mAh one.
The usual warnings apply: check in advance with your airline, rules and regulations vary around the world and so on.
Ah, great choice! I use the same model for my NAS. Anyway the N100 is really powerful for this task, and should have enough spare power to allow experimentation with different addons. Plex likes the intel chip and uses it for transcoding, if you want to do that.