The optimal number of tags per icons is between 4 and 7.
Less tags is better, which means that icons with 3-4 tags will rank higher than icons with 7 or more tags.
In order to combat keyword spamming, our algorithm applies a "dampening effect" on icons that have more than 7 tags which means it is reducing the search score for those icons. The points-reduction is very small, but increases rapidly for more than 7 tags.
The aim is to prevent having icons with as many tags as the designers can think of because that will cause spamming of the search results with irrelevant icons.
Sometimes, it's better to rank a little lower to show up on more high-value searches (more specific). For example, it could be that many business-related icons will have more than 4 tags to be distinguished from others. However, most of your icons should ideally have 3-4 tags.
How about 1-2 tags? 1-2 tags is generally too few. However, 1-2 tags can be okay in circumstances where that one word is the most logical. For instance, for an avatar icon of Gandhi, just having the tag "gandhi" would be fine because 99% of people looking for an icon of Gandhi would just type that.
For further information on tagging, check the following articles: