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.