The number is added to a SEO name if the same name is already in use for another object (product, category, etc). This was created so that if the administrator gives the same name for two categories (products, manufacturers, etc), the system adds en (or another current two-letter language code) to SEO name of the second category automatically.
For example, if the administrator gives the SEO name Books to a category аnd then gives the same name to another one, the system adds en to the second one. And after that, the first category will have the Books name and the second one will have the Books-en name. And if the administrator gives the SEO name Books to one more category, Merchium starts to add language code+number to the category SEO name: Books-en-2, Books-en-3...Books-en-17, etc.