It is a complex algorithm. The fact that there are only eight symbols per card with one identical symbol is what makes the game so much fun: it is quick and exciting. The manufacturer came up with a new variation that they have not yet put in game boxes. They have it posted on their website, it is a blast!
There is a theoretical maximum number of cards that can meet this constraint, for any N images per card the maximum number of cards (and images) is (N-1)^2+N, which for 8 is 57 cards/images, if all 57 cards were present then each image would appear 8 times across all the cards. As there are only 55 cards in the deck then 14 images only appear 7 times and 1 image only appears 6 times. Why they choose not to provide the maximum cards is a little strange. Exercise for the reader construct the 2 missing cards :)