Number of keywords

You begin to understand keywords better when you understand the way buyers search. Which engine are they using to find your image? The stock site itself or through another search engine such as Google? Many buyers come through the search engine first which isn't looking at keywords; they are scraping the title or caption. You really need to have a great caption and/or title and then only add keywords that help extend your caption. Buyers arriving at the stock site to search will see images returned first that are likely moved up in ranking based on how often they are sold. But the stock site will always find a way to show new photos to the buyer in the first batch or page so the results look more extensive and to give more contributors a chance to make a sale. Your keywords are now very important and need to be accurate as results will be attached to the most relevant keywords. To get pushed up to the higher search results, use what's been termed "long-tail" keywords. You can look into that for some tips. My overall advice is that adding more or less keywords to reach a quota is not the way I recommend approaching the puzzle.

It's better to have 15 relevant keywords than 50 irrelevant ones. It appears that the search algorithm is built in such a way, that the distance of the keyword to the top is important. So, the 50th keyword has probably 0 to no influence on the search results, especially in a crowded field.