A couple of weeks ago Xpiks posted a pretty good guide about microstock keywording on their blog. It's a good read if you are looking for some tips on increasing your sales by optimizing your keywords. It has a link to a case study about the total keyword count of top selling media, and this article makes a good point about the results of the data in that study.
How many keywords to add
The only correct answer to this question is “enough”. Enough to serve the purpose of keywording - targeting a certain group of customers looking a certain type of media. “Enough” can be just
10keywords or it can be the
50. Don’t aim for the higher number if you don’t know what to add - agencies “punish” keywording spam.