I'm going to add that the math about the Deck having a 5200mah battery and thus with (let's say) a 25000mah powerbank it's easy to think that 25000/5200 almost 5 charges... that's wrong. The Deck has 7.7v battery while powerbanks normally are 3.6 so things don't translate directly.
Fastest way to know? https://www.steamdeck.com/en/tech/ The Deck has a 40Whr battery, a 20000mah powerbank should be around 74Whr and there the math is clear: less than two charges. Usually on the bottom of a powerbank there's the specs, should have that value too.
You shouldn't have to do any conversion, the whr value is what's used to allow or not powerbanks on airplanes so that value is either clearly in the specs, or if you already have it, on the bottom :)