You can see your average no. of posts per day by going to your profile and clicking on the statistics tab. I'm afraid if you want to know the total number of posts you've made on a specific day you'll probably have to count them though. You may be able to use the advanced search feature to help you by searching for all posts made by your username on whatever date.
We used to have a top 20(? not sure of the number) posters list but I don't think we have that on the new board. Someone might know better than me though