I have a 6 cup Moka pot (about 250-300 ml) but I want to prepare a single cup of coffee just for myself. How should I reduce its load - reduce amount of coffee beans and amount of water or reduce only coffee amount? Will this reduction affect taste of prepared coffee?
The Moka pots like Bialetti are very similar to the esspresso process except that the water goes through the coffee from the bottom in Moka's case.
They are designed to match the pressure of the water with the resistance from the coffee grind/puck (IIRC 1-3 Bar of back pressure). If you use significantly less coffee, the water can just run through the coffee compartment without extracting much flavor or worse, it can fill the compartment (replacing the missing coffee) and soak for too long.
My guess is that you'd need to reduce the volume of the compartment to allow proper pressure build up on the coffee. Perhaps roll a coffee filter into a ring shape and place inside the coffee compartment to take up the missing 5 cups worth of coffee grind and fill the inside of the ring with the now less coffee grind.
Again you may not be able to reduce the water all the way since it needs to be above the opening of the spout/pipe that descends into water reservoir. The solution here is to take the Moka off the heat when you're near your one cup so you don't dilute your Moka Espresso too much.
You should reduce both the amount of water and coffee beans. The exact amounts depend on your preference, but if you know how much you use for a full pot, divide both the water and the coffee beans by the same number. The ratio should stay the same.