I was reading earlier that the optimal number of votes we should do each day is 20, and I'm wondering if there is any similar kind of number we should aim for as far as the number of posts(excluding comments)? How many posts per day would be too much or too little?

I've seen people say that if you want to build a steady following you should post daily. I've seen others say doing that is over exposure. Guess it depends on your goals. My average has been about 1 a day thus far with an occasionally 3 day hiatus.
Don't post more than four if you want the maximum output!
This is a good short answer. After 4 posts in 24 hours there is a % penalty applied off the top of the payout.
As others have said 4 posts a day max but you can do 40 votes. Just keep an eye on your voting power on steemd.com or steemstats.com.
Things change on Tuesday thought so keep an eye out for the updates.
If you post more than 4 the payouts go wonky!!! Technical talk here not included. It has something to do with stopping spam, and spreading your vote rewards out.
I think the limit is whatever you feel comfortable with. Some people post a lot because they have a lot of different interest, some people only post to a certain crowd while the avg user post in variety, but all in all I think the number of post put out per day its a trait and depends on the person and what messages they are trying to convey.
Good Post! I hope I wasn't confusing.