The pacing (the speed at which new discussions appear) needs to suit the community's stage in the lifecycle.
In inception-stage (new) communities, the pacing needs to be much slower. If you've just launched a community, you should only post a new discussion when the last one has received at least some response.
Over-eager community managers can post 10 new discussions a day and get 5 responses. A response or two every looks bad. It deters people from participating. As the speed and quantity of responses increase, you can speed up the frequency of new discussions.
However, if you initiate too many discussions without waiting for any to get a response, you risk having a community that looks very empty. If you post a discussion which receives no response, either remove it after a day or two or individually prompt people to respond (this might indicate bigger relevancy problems).
In mature communities the pacing of new discussions will naturally be faster. You need regular members to frequently see new discussions they can participate in. Sometimes that takes care of itself, quite often you need to initiate a discussion or two to fill the gaps.
It's a fine balance.