I think both of those are fraught with danger.
If you only allow a limited number of users to respond you’ll tend to hear from the loud keyboard warriors who can spend lots of time online and not hear from the quiet knowledgeable people who can only spend a limited amount of time online.
Equally, if you limit the number of replies thatcan be made, then you can lose excellent insight, e.g. I’ve been reading along and have had my 2 allowed replies to clarify what people are saying and then I have the breakthrough thought that has the answer to life, the universe and everything, but no-one will ever hear it because I’ve hit my limit. Sure I can start a new topic, but it’s out of context and even if you point back at the previous topic how many people will read that before replying to you?