The numeric indexes (-2, -3 and so on) are natively appended to WordPress permalinks if the slug is used more than once. For example if you create a page or post with the same title, one of the permalinks will be changed to make the slugs unique:
https://example.com/example-title (Post) https://example.com/2018/11/28/example-title-2 (Page) https://example.com/products/example-title-3 (Product)
It is possible to remove them with Permalink Manager with just a few clicks. Firstly, you need to enable “Force custom slugs” in “Tools -> Permalink Manager -> Settings”.
After that you will need to reset the permalinks using “Tools -> Permalink Manger -> Tools -> Regenerate/reset” (bulk reset) or individually (using “Restore to Default URI” button inside Edit URI box on “Edit Post/Page/CPT” admin page).
To make sure that no permalink is duplicated please go to “Tools -> Permalink Manager -> Tools -> Permalink Duplicates” section.