You may answer as many as you wish. This process of responding to statements should continue during the discussion period. You do not have to answer each and every email. There are too many students for this. However, you should try to participate throughout the discussion period.
Quality is important.
It is fine to agree, but you must add facts to further support what you are agreeing with if you wish credit. When you write your emails, re-read them. They should read as discussion. They should say what you would say if you were having the discussion in a classroom. They should be as brief as possible, while getting your point across.