As per the ancient Greek tradition, putting on candles on a cake is a special way to pay tribute to their Goddess - Artemis. The cake was thought to represent the moon and as a result, lighting candle means that such people believed that these candles carry their smoke and best wishes or prayers directly to the Goddess who lives in the skies.
Similarly, these candles carry a specific power that is believed to bring a respite from the evil spirit that visited the people during their birthdays. As a result, these protected the birthday guy against evil and partygoers thus try to make them merry. The laughter and sound of such partygoers is believed to scare away the spirits.
So, how many candles are okay on a cake? The same can be dependent on the age of the person and if you are beyond a certain age group, it is best to opt for single candle.