How many watts??

 If I may add to ^ @liquidsound 's post.

 Other considerations include room gain, which most will appreciate as an apparent increase in volume for a given power output.

 And that many loudspeaker's sensitivity is rated as 2.83V/1M rather than 1W/1M. This will mean that with each halving of impedance, the loudspeaker will lose 3 dB of sensitivity. For example: @ 8 Ohms 90 dB,  @ 4 Ohms 87 dB , @ 2 Ohms 84 dB. To ensure linear frequency response at high volume levels, it behooves one to be sure that the accompanying amplification can meet the concurrent doubling of power demands beyond the standard 8 Ohm power recommendations.

Many amp designers say amps tend to work at their most linearity at around 20% of their full rated output. As has previously been posted, volume increases require logarithmic power output. Having an additional 80% of total power output for headroom is prudent. Running out of power; can lead to clipping; clipping can lead to speaker damage.
Speaker sensitivity, room considerations and desired volume levels will ultimately determine how much power one needs.
There used to be an oft repeated audiophile general rule of thumb; "double the manufacturers minimum power recommendation".