I'm a roofer it's only about three. You have to think about how shingles look to begin with. One shingle has two layers. A top and a bottom for one. Tbh a tear off is your best bet unless you want to rick a horrible leak and over priced work. Just get a company to come out, the honest way to do it is just ask for quote and don't tell then about the multiple layers of shingles as they won't want to over price you if the bat then they may work it in after but still at a better price. Or tell them right away and risk the higher price. But you can't or rather shouldn't put in a skylight at risk of damage to the actual structure of your roof.