Well, you need a multimeter or other means of testing the resistance (Ω) of your
coils. From there, if you wrap a single coil at 1Ω, it will read as 1Ω. If you do 2
coils that would each be 1Ω on their own, on a dual coil, it will read as half that much resistance, in this case .5Ω. Your wire is like a pipe, the smaller it is, the more it slows down the electricity in it. The longer the wire, the more it slows down. That's where your resistance comes from. Longer wire, more ohms, thinner wire, more ohms. If you do not have a means to test the ohm's on your build, do not fire the coil until you do. If you know your ohms, plug that and 4.2V into the calculator I linked above, to make sure you're within your battery's limits.