Maybe you'd need 16 layers of drywall all put together in one monolithic slab to get an STC of 45, but on a standard wood stud wall, you can get STC 45 with 2 layers of 5/8" drywall per side, with absorptive insulation in the cavity. Without any insulation, you'd need 4 sheets of 1/2" per side - 8 total (the wall would be 7 1/2" thick).
Adding resilient channels, staggered studs, steel studs or double walls will raise the value even more.