A few rules to makes things a little easier to understand.

Number of Characters.
There is no definitely limit on how many characters you are allowed, although keeping five characters active would be a challenge. It is recommended that you have two or three so that you can have one character in any situation.

Number of RP's allowed to post in.
Again, there is no limit except your own creativity, just make sure that whatever thread you do join you keep active. The responsibility to keep a thread active is up to you.

Players allowed within a thread
You cannot stop others from being within a thread, you can ask them not to because of a certain reason but that reason must be good. Exclusively two player threads are not allowed in the Public Area.
I can understand if you wish to make a thread in which two characters meet together or if they decide to mate, just don't keep others out for the entire thread because what point is there in having this happen if others can't enjoy it as well?

Players not posting within a thread
If a player does not continue to post after a warning post then you may skip them once after a small time has passed. After all, we all have lives outside of SF as well. If this continues for another round then you may give another warning post, wait as long as an appropriate time has passed then skip that player. If this happens a third time then you may skip that player indefinitely.