If you count raw number of features edited, it's way easier to crank up the number by drawing some buildings, forests, lakes, landuse from satellite/aerial imagery.
Whereas things like points of interest (shops, services, tourist-related stuff) won't get you a high number, but arguably are more important.
Corporate editors in under-mapped countries certainly can add much more data than average mappers do.
On the whole it's hard to measure the impact someone has on the project though. Some people tirelessly keep the documentation wiki up-to-date, or paint bikesheds on the tagging mailinglist. Some fix bugs in JOSM, Carto, and anywhere that comes along their path. Some manage to get governments to agree to license valuable data sources for use in OSM (this one is potentially huge), and again others write JOSM plugins to unlock such data or to scratch a common itch improving the efficiency of hundreds of mappers.ยน
There is one mapper in the Netherlands who welcomes all new mappers starting of in the Netherlands: that's an invaluable service hard to quantify in data!
1: Or in my case, build a plugin that a dozen or so folk seem to use once a month. :P
One great metric is mapping days (distinct calendar days in which the user was active) which sort of abstracts the fact that different people tend to have a different idea how much work warrants a single changeset.
- Focus on specific regions
- Both creation and deletion of map features, but more creation than deletion
- Unique and descriptive changeset comments
- Engagement in changeset comment discussions, both of your own and others' changes
I made edits changing one tag that were far more useful and time-consuming than my other edits that added 9 000 objects.