Okay everyone with a post count of less than 50 has now 3 lead and 5 reply limit. Everyone with a post count of more than 50 has unlimited. I put in 9999 but I doubt that anyone will be able to make that many posts in a day.

If you click on "lists" you will see that I have made two lists. One list has all members with less than 50 posts and the other has all members with more than 50 posts.

Less than 50 posts: everyone has 3 leads and 5 replies. More than 50 posts: everyone is virtually unlimited.

This is what is going to happen: when new members start posting on your board, they will automatically be added to the less than 50 posts list.
When people reach more than 50 posts, they will automatically move to the more than 50 posts list.

However, read carefully. When new members get added to the lists, whether it be the less or more than 50 post list, they will NOT automatically assume the post limit. Say a new member starts posting, this member will be on the less than 50 posts list, but it doesn't mean that they will have a 3 lead and 5 reply limit.
When a member goes over 50 posts and gets automatically moved to the more than 50 posts list, the post limit for this member will still be 3 lead and 5 reply.

There is only one way around this that I can think of.

Click on "security" in your admin menu.
Scroll down a bit and add 3 and 5 to the daily lead and reply in the appropriate boxes.
Save the changes.
Now everyone who starts posting on your board will have a limit of 3 lead and 5 reply until you change it.

Then once a week or so, click on "lists" in your admin menu.
Click on "edit members" in the list that says 50 and up
Reset the daily limits to 9999 or unlimited.

That would be like a weekly update in case someone goes over the 50 posts and gets added to the list.