To give a serious answer: As many as the story requires. The same thing goes for any ethnicity. If neither the story nor the character nor any of their dialog require it, not describing a character by their ethnicity is a valid (albeit somewhat harder) choice. This way, anyone can read and imagine the story with what they are familiar with. Now don’t get me wrong, you can absolutely assign every character a full set ranging from emotions and values to physical attributes and ethnicity - but you don’t necessarily have to state that “Jade” has dark/light skin. Simply describe the character on a different level. This is complicated, but beautiful if done with cultural identity: Someone from a community of turkish guest workers may have a very pragmatic and hands-on approach at their job but be somewhat hands-off in the household, until they have guests (Chosen from an arbitrary pov, this is not grounded in experience). If you wish to determine what ethnicity a character has, first ask yourself: is it important/does it influence them? If no, try to leave it out maybe? If yes or you absolutely want to know it, rolling dice is a valid option: Check the distribution in the chosen community and simply roll. From what I know many authors base characters, settings and scenes on some kind of real-life example, so naturally one might base the ethnicity on the same example.