Small-ish power amplifiers, like 100W would do many headphones at once, or big and do a lot of headphones at once. Use an attenuator I each headphone, it can even be adjustable.
Not kidding, this is widely used in recording studios, a signal is fed from a few power amps to all places and then a box would select the source (from a stereo connection, select stereo, mono only L or R) and attenuate.
You have to do some math to calculate the attenuator resistor for the given power and impedance and then estimate how many HP would run from one. It's not a very efficient way, but the more quantity of smaller amps you use is more efficent. Or, if you need efficient, you could use a big ass transformer in the output of a big ass power amplifier and get an output voltage closer to what you need, then a small attenuation or level control in each seat should be enough.
JS