Question: How many instances should there be under each plot element?Answer:
Assuming you are referring to the 8 basic plot elements...
There is no rule about the number of times each element should be illustrated in a story.
The basic principle is that each element should appear at least once for the plot to feel balanced. And once is enough.
However, it is pretty common to have several forewarnings or requirements. Some writers like to have these illustrated once per act (three or four times). Same with most of the other elements. This makes for more of an emotional roller coaster, in which the reader oscillates between hope and anxiety for the characters.
Obviously, you will have only one goal and consequence, but you may remind the reader what these are more than once, and especially if it feels like the story is getting off track.
If you're just brainstorming story ideas, it is a good idea to write down as many possible illustrations as you can under each category. Later on, you may narrow these lists down to the best choices.
And, of course, you may discover better illustrations during the actual writing process, and should feel free to revise as long as it makes the story better.