Welcome to Steemit. As far as I know, Steem is not vulnerable to the type of Sybil attack that you are suggesting (except possibly for gaming the hot or trending algorithms). Essentially, the total number of votes is irrelevant. What matters is the stake-weighted votes. That's why one vote from someone with a significant amount of Steem power could be worth hundreds of dollars, while it could take hundreds of votes from newbie accounts to reach $1. Some people keep their Steem power distributed among different accounts, and program them to automatically vote on the same posts as some 'leader' account. The amount of rewards that one post receives will be identical, regardless of whether someone votes with all of their Steem power in just one account, or if they distribute the Steem power to multiple accounts and vote with all of them.

Hope that helps.