Hi! Sorry about the delay in getting back to you about the minimum number of tags.
It is allowed to have icons with 1 tag, but it is generally not recommended. We will start showing warnings in Sales Analytics for those sets that have less than 1.5 tags on average, among all the icons. So, we will look at it at set level and not at icon level anymore. This will make it more flexible for designers.
These changes are not yet operational, so you might still be getting some warnings for individual icons until we implement these changes.
However, remember that tagging is very important for the discovery of your icons. It cannot be taken lightly.
You can add 1 tag for the icon and the icon will appear in 1 search. That will give the icon low visibility and therefore, lower possibilities of sales. Using 1 tag makes the most sense for cases such as the Iconfinder logo or Facebook, where it would be enough to tag with only "iconfinder" and "facebook" respectively.
You can add an optimal number of 3-4 tags and increase discoverability. However, tags should be kept as relevant as possible. We have this article that explains our updated tagging guidelines more in-depth: