Medium format film is pretty genius in this way:
The film you load into your camera is already wound around a take-up spool. When you advance your film from left to right (or bottom to top, in the case of the Lubitel 166+) inside the camera, the spool from the film you’re shooting becomes empty, and becomes the take-up spool for the next roll. As a result, you will always be left with one empty take-up spool in your medium format camera – just move the newly-emptied spool to the opposite side, and you’re ready to go!
All take-up spools look pretty much the same and all function exactly alike. If you think you might lose yours, simply visit your professional photo lab – plenty of extras can be had, and usually for free!