How many Watts? That depends...
Wattage capability of the speakers?
Looneybomber addressed that - add the mfr's rating if using dual drivers, and then take a big dose of windage as you really don't know if the mfr's rating was with highly variable program (music) or test tones.
Watt's to hit a sound level?
Poorman's getting at it. You need the speaker sensitivity, the room size, and where you're sitting relative to speaker placement. From there, it's your choice of average level and headroom.
AVR power a good match to the speakers?
The first point is what most folks think about, ratings comparison, and they're wrong in thinking that "the speakers are rated higher than my amp, so I won't blow speakers." That's looney's point... there are stress conditions where an amp can blow a speaker rated higher than the amp.
The useful answers start with "how loud can it get" but only if you ask:
"how loud can it get at 0.1% of available power."
- 0.1% because you want 30dB of peak headroom on the average amp output
- "available" power because we're talking peaks, and peak power output is well above RMS ratings.
Keep in mind that this is still only half the answer; how loud you want and room size are big factors and speaker sensitivity is the critical parameter. That's your call, but these are some guidelines. Just trust your ears and turn it down when it starts to sound bad.