Sybil attacks are carefully considered in the protocol design: Cardano Community Meetup in London: Ouroboros PoS Research - Prof. Aggelos Kiayias - 1:11:38
Basically, there will be a mechanism that deincentivizes Sybil behavior by making it expensive.
One way to prevent this is to identify the real-world persons running the pools, so that we know whether whether they are running multiple pools.
Even if you identify everyone, how would you know a subset of the people aren’t secretly a cartel?
number of pools is targeted to be around 100
According to the latest data, we can expect around 1000.