It depends.
On room size, how loud you listen. That's about it.
Realize that power alone is not always the answer. The amp has to be able to deliver power smoothly into the speakers variable impedance at different frequencies. Amps that double in power from 8 to 4 to 2 ohms do this best. Some speakers are more difficult to drive due to complex impedance loads at various frequencies. Others are less demaning.
On room size, how loud you listen. That's about it.
Realize that power alone is not always the answer. The amp has to be able to deliver power smoothly into the speakers variable impedance at different frequencies. Amps that double in power from 8 to 4 to 2 ohms do this best. Some speakers are more difficult to drive due to complex impedance loads at various frequencies. Others are less demaning.