Complex sentence: Behind each of the numbers was a card, displaying either the name of a prize or the word "WIN" were wild cards and if one was uncovered, the champion chose one more number and won whatever was on the card behind it.
Split into simpler sentences: Behind each of the numbers was a card, displaying either the name of a prize or the word "WIN". The lattter cards were wild cards and if one was uncovered, the champion chose one more number and won whatever was on the card behind it.