Voltage source for single transistor audio amplifier
Answer: 20V

Simple (very crude) approach, say when collector resistor is given (not asking why is it 10k): Transistor power limit -> reduce to 1/10 for safety -> choose collector current limit -> divide power by current to validate to 1/4 of max collector voltage -> multiply by resistor value -> double it to keep output in the middle -> get voltage. 

The datasheet says: ~625mW is a limit, then 62.5 mW is nominal. Then choose sensible collector's current. Looking at datasheet it is between 1..10mA. Looks like for ~50mW the CE should be around 5..50V. Validating the CE limit from datasheet 40V/4=10V. Deriving for 10K resistor and voltage of 10V getting 1 mA current. Doubling it -> 20V.

No matter how safe it all looks, the number looks too high and overall the cascade feels a bit fragile. I'd rather go with no more than 9-10V and reconsider output impedance and optimize on something like bandwidth or noise figure.