I'm working on an engine for Win32 exclusively. Can I safely assume that the flag VK_PRESENT_MODE_FIFO_KHR will always generate 2 and only 2 images for the swapchain?
There is nothing in Vulkan which guarantees or requires this. So even if it were a safe assumption today (if it were enforced by Windows in some way), all it would take would be one Windows update to break you.
It's best to work with what you're given, even if what you're given may be more than you would prefer.