Determine which cells of your tables contain formulas. You might want to mark these with a colour fill to be able to locate them later.
Also mark all the cells containing labels, intended to be kept.
When you have marked all of the cells whose content you want to keep, select the cells with entered data that will be replaced with new data when you re-use the table. Delete the contents of these cells.
Check for other cells with content you will not need in future documents made with this spreadsheet. Delete that content as well.
When you have removed all the data to be deleted, and have kept all of the formulas and labels that are to be kept, go to the File menu and choose Save as template…
You'll be presented with a choice of where to save that template:

You may want to keep it in the folder you are using to keep the completed versions of the documents, or you may prefer to keep it in the Template Chooser with other templates. Either will work. In the chooser, Your template (and others created by you) will likely be placed in a 'My Templates' section. You may get a choice of what section you want it placed in, but this was not an available choice in earlier versions of Numbers (or Pages) where I've saved templates, and I'vee yet to do that in the current version.
Once the template is in place, you'll have a 'permanent' clean copy of the document to use each time you need a new document.
Regards,
Barry