IDirect3DDevice9::GetNumberOfSwapChains

DX9 vtable[15] → UINT

IDirect3DDevice9::GetNumberOfSwapChains is a method of the COM interface IDirect3DDevice9 in Direct3D 9. Its function pointer sits at vtable index 15 — the zero-based slot in that interface's virtual method table, counted from QueryInterface and including every entry inherited from a base interface. It takes no parameters and returns UINT.

COM methods are reached through that table rather than by an exported name, so the slot index is what a wrapper, an interface hook or a disassembly listing has to resolve against.