The basic unit of stamp collection in traditional philately is

The simplest one at the beginning is to collect the stamps on the actual mailing envelope. The unit should be called "pieces". When I was a kid, I would compare who had the most stamps by the number of stamps; after taking this a step further and philately becoming a commercialized activity, the basic unit was a "set"; in the future, for stamp collecting activities for exhibitions, the unit could only be a "piece" Damn it, because he is a collector of a certain topic, and there are many such collections.