There is no magic number, it is just the more you vote, the more your voting power dillutes to the many posts you vote for and if you have less Steem Power, then each of your votes will not count for much and you will hardly get curation rewards back.
Yes, you don't get the full 100% of the reward for a post you make.
So 4 posts a day and 20 - 30 upvotes. Do you have any idea what the magic number for comments is? I hope those don't count against your post number.
There is no magic number, it is just the more you vote, the more your voting power dillutes to the many posts you vote for and if you have less Steem Power, then each of your votes will not count for much and you will hardly get curation rewards back.