how do I print ONE page of a multipage document ?

With Pages 10 on Mojave, you select the specific page thumbnail, and then command+P to open the print panel. By default, All pages will be selected, and you can then change that to From 3 to 3 if you wanted to print page 3 of your document.

With Pages 10 on Catalina, you select the specific thumbnail of the page to be printed, and then command+P to open the print panel. Apple has changed the interface, so you will see Pages: [ All ]. Other choices are Single, or Range. Select Single, and an entry field appears where you either directly insert the page number, or click the increment until the page number is reached. These are not the logical page numbers that may exist in your document, but rather the actual thumbnail number.