The above reply is correct. You can go to the ETF website and look at what it is composed of, there are stocks or bonds or whatever that make it up. The annual reports will list the actual number of stocks for each company.

Passive ETFs try to match a particular index, so as ETF units are bought or sold, the ETF issuer buys or sells proportionately or match the index. It is up to the investor to decide whether they have faith in the index the ETF tries to track. I believe ETF issuers typically do a quarterly rebalancing as well. Things get messy when ETFs shift to a different index. That does happen, larger ETFs will carefully shift over to avoid affecting the stock prices.