Since the paper is short, you will not conduct an in-depth research, but will rather produce a summary of some topic and your opinion/attitude related to this topic.
That is why many undergraduates have to write such essays very often.
There is not a specific number of sentences that such a paper should contain.
This size of a paper is not very small and requires much effort from you.
If the teacher asks you to write a 500-word essay, you should do it right and use the correct formatting, methodology, and develop a correct structure for your paper.