Reviews appear on pages of products they do not concern

1. In my example, on the subpage of a specific paint, I see a review of another product Why is this happening?
2. What do these numbers in brackets mean? I have attached a screenshot.

1. Have you enabled the "product_reviews_by_category"? It's an extra vmod included that makes the display the reviews for all products in the same category.

2. The number in brackets is quantity.

Yes I did
This explains it all and overall it's a good idea when the product has no opinion yet.

Is it possible to make it so that when a product receives the first opinion, opinions about only this product will be displayed?

Then reviews matching the category from other products will no longer be displayed.

It is possible, but not covered by the default addon functionality.
You would have to produce a couple of conditions in the vmod code.