The only limit on the deck size is that you have to be able to shuffle it on your own without assistance, and that your sideboard can be no larger than 15 cards
Then if you cannot you'll lose the game
However, don't expect to find serialised cards, borderless frames, or that one-of-one The One Ring in here
Its a good place to run March of the Machines for infinite ETBs
Check out Popverse's guides to: About PAX West A Celebration Of Video Games, Tabletop, & Game Culture
On the other hand, in general, the partner pairings score low, with one exception: the Shark / Human combo of Shabraz, the Skyshark / Brallin, Skyshark Rider has 141 decks (which is especially surprising since Shark wasn't a creature type in Magic before Ikoria and Commander 2020 ), making it the most popular partner pair by a significant margin