When you open a template a new document opens that's based on the template you selected. That is, you're opening a copy of the template, not the template itself.
That’s the template's special power: it opens up a copy of itself, imparting everything it contains to a new, fresh document. You work in that new document, benefiting from everything that was built into the template, plus adding or deleting what you need to. Because the new document is not the template itself, your changes are saved to the document, and the template is left in its original state. Therefore, one template can be the basis for an unlimited number of documents.