Light Up The Night

If a woman is lighting more than one light for Shabbos, why does she recite “L’hadlik ner shel Shabbos”? Ner is the singular form. Shouldn’t the correct text be l’hadlik neiros? The Pri Megadim says that it is proper that no matter how many neiros a woman lights, she should always recite “ner shel Shabbos.” This is because the mitzvah of Shabbos lights is fulfilled by lighting just one. Indeed, the Mishnah Berurah writes that one who has very limited funds should light only one, and spend the rest of the money on appropriate Shabbos food.
Why is it universally accepted to light more than one light for Shabbos?
Rebbe Shimon bar Yochai and his son hid in a cave for many years to escape the death penalty decreed by the Roman government. Upon exiting (the second time) they witnessed an elderly man carrying two bundles of myrtles. They inquired of him as to the purpose of the twigs. The man replied that they were to be used as smelling spices on Shabbos. Whereupon Rebbe Shimon bar Yochai and his son asked, “Wouldn’t one bundle of myrtles suffice?”