When images are uploaded to Picfair, they can be purchased as prints or digital downloads an unlimited amount of times, and it isn't currently possible to sell your images as limited editions.
However, you can remove your images at any time, therefore if you only wanted to sell your images a number of times, you can delete them from your store and profile once they have reached a number of sales, therefore limiting the number of times the image is sold on Picfair.
You can see how many times an image has been sold from your dashboard, here.