Typically, you need to know at least two things to evaluate power. The impedance controls the overall voltage level needed to put in enough milliwatts in to the headset as function of voltage, P=U²/R, and you are probably best off looking up the actual impedance measurements rather than using some single nominal value given by manufacturer. It is also of interest whether the impedance varies by frequency, and if so, by how much. The other is, what sound level is achieved per milliwatt of power, or per 100 mV of driving voltage from peak to peak, or however they prefer to express it.

Power requirements of headsets are generally low in context of the systems that are tasked at driving them. From what I have seen, the range is somewhere from less than 1 mW to about 30 mW for something like 96 dB SPL which should be loud enough to cause hearing damage if prolonged. I am not sure if there is use case for having much more loudness than this, but watts multiply very quickly when target SPL goes up given that this scale is logarithmic, but on the other hand, so does driver excursion and thus distortion. Many headsets are operating close to their maximum performance at 96 dB and usually only magnetoplanars and electrostatics can handle more SPL without distortion going out of control.

If the impedance of the headset is low, it should work perfectly with the $10 Apple USB-C DAC dongle, as this particular one provides low-impedance output at mere 0.9 ohm, which is exceptionally good, and has been measured to be capable of supplying 33 mW of power to a 32-ohm load before distortion. (The issue with high output impedance is that it adds a frequency-varying component which distorts frequency response if the headset's impedance also varies as function of frequency.)

The other issue is whether it gets loud enough for you. This is question of providing high enough voltage to drive the headset. This can be a problem with high impedance headsets, as they tend to require much larger driving voltage because their large internal resistance forces the power to be very low until something like full volts are hit. For instance, if your headset had 600 ohms impedance, and it takes a milliwatt of power to drive the headset loudly, you need to provide something closer to a 0.8 V (P = U²/R = 0.8 V * 0.8 V / 600 ohm = about 1 mW), whereas most low-impedance headsets are already very loud at mere 0.1-0.2 V output voltages.

I personally go by audiosciencereview's measurements as they typically add each headset to their list they put on every review and show the millivolts needed to reach 96 dB benchmark level. Some low-impedance headsets like Hifiman HE6SE v2 with 64 ohm impedance actually require over 1V before they get loud, so it can be said that some planars are very inefficient, but even that is relative to other headsets, as it is still a few dozen milliwatts, similar to the power requirement of a single standby led you see all over the place. Most are not that bad. So the summary is probably that you can simply try driving with the Apple USB-C DAC dongle first, and if it is not getting loud enough, then get something else with more output voltage.