Any 'rule of thumb' relating loudspeaker wattage to audience size is either wrong or at best an over-simplification! Wattage alone does not provide quality or loudness. A set of ground rules for loudspeaker power is proposed below, but first some basics.
People absorb sound and generate alternative sound. Hard surfaces reflect sound. Loudspeakers have varying frequency-dependent efficiencies. There is a helpful noise chart at Noise Help. It indicates that human ears can tolerate exposure to 91 decibels for up to 2 hours. This is a reasonable maximum.
Loudspeaker specifications should indicate their sensitivity as sound pressure level (SPL) in decibels (db) measured at 1 metre with 1 watt applied. (The range of my speakers is 88-98db.) Doubling the wattage increases SPL by 3db. Doubling the distance to the listener decreases SPL by 6db. The critical distance is between speaker and furthest listener.
1. Minimize the sound absorption of the crowd by positioning mid- and high-range speakers well above head height and angling them down to point at distant head height. That ensures loudspeaker sound can travel direct to audience ears and not be baffled by bodies.
2. No matter how loud the crowd noise is, the SPL at their ears should not exceed 91db. We can use Geoff The Grey Geek's calculator to determine RMS speaker power for various distances (see table of results for common distances below).
3. Always check sound levels with a Sound Level Meter.
Alesis Monitor One Mk2 Passive (studio)
Wharfedale Titan 12 Passive (mobile PA)
Maxima for Titan 12s
Ensure that the maximum power rating of loudspeakers is complied with! If greater distances than these are required, additional loudspeakers would be better than more powerful ones, as the objective is to provide music and not deafen the audience!