How many sentences should a conclusion be?

It depends on what the conclusion is for.

In middle/junior high you can maybe get away with three sentences.

In high school it is expected that your body paragraphs are more in depth, so the conclusion should be at least five sentences.

If you are in college then you shouldn't even be asking this question.

As a general rule, I would recommend 5 sentences for a conclusion.

Source: My essays are always awesome