Here are four lines of text with three characters in each line.
Here are three lines of text with five characters in each line.
It means no more than 120 lines of text with 80 characters in each line. (If you use "Courier" font, you can set your margins so only 80 characters will fit in each line to make it easy to see if your essay fits.)
Spaces between words count as characters. Blank lines between paragraphs count as lines.
As another perspective, my essay was 35 lines of text, 34 if you don't count the spacing between the paragraph, and it was just over 500 words. Roughly speaking, you can fit an essay of about 1750 words.