This is how to write a summary of any piece of written work: an essay, a chapter of a book, a short story -- anything.
a. Count the number of paragraphs in the work.
b. Estimate that each sentence you write will contain about 12 words.
c. Write down the required length of your homework assignment in words -- the number of words you have been assigned to write.
For example, if there are
then you will need to write about 12 words on each paragraph (c/a = 11.67).
Since an average written sentence is about 12 words long, that means you will have to write about 30 sentences, or 1 sentence per paragraph.
So, if you have been assigned to write a 350-400 word summary, you can write up to 440 words (110% of 400).
So, if you have been assigned to write a 350-400 word summary, never write fewer than 385 words (110% of 350).
This is a guideline, an average: 1-2 sentences per paragraph.
The longer the piece of writing you have to summarize, the more sentences you will have.
But no matter how long the material to be summarized is, you are going to have to write about one sentence minimum on each paragraph, and sometimes two (or, occasionally, even three) sentences on each paragraph.