It's usually done by listing our top 20 housemates in order and assigning twenty points to whoever the voter puts 1st, 19 to 2nd, and so on (1pt to 20th).
Maybe you could limit the number of housemates per series people can vote for, though? Say... 4 or 5?
But then I'd want to vote for like 8 BB6 HMs
LOL