How to find out how many characters are in the key? Again about the thermometer of tonalities ...
In general, the number of key signs and the actual signs themselves (sharps with flattenes) must be simply remembered and simply known. Sooner or later they are remembered automatically - whether you like it or not. And at the initial stage, you can use a variety of cribs. One of these cribs on solfeggio is a thermometer of tonalities.