The FAQ is specific about this: It is not expected that players will vote in every match. You are free to vote in as many or as few as you like, the only rule being that you abide by the honor system (i.e. at least try both games in good faith for a reasonable period of time before voting).
Using the Free IF Playofffs as a comparison, some early matches had as few as 7 votes total. The tournament will work fine if you vote on the games that you already know and/or choose to play in good faith.
So: There aren’t really expectations, though the purpose of the “quiet play” period and the extended duration of initial rounds is to allow more people to complete more games.