If there exists any limit on how many topics you can make in a certain time period, I've never heard about it. Either way, imposing a hard limit of 3-5 threads per day on all users (possibly varying for paid subscribers) would really cut back on superfluous topics, especially in System Wars. For example, this user has posted 14 SW threads and counting in the past 3 days, which I think anyone can agree is excessive.
EDIT: Now 19, and counting. 10 threads today.