How many watts of amplifier do I actually need for my subwoofer?

Match the amp's RMS rating (not peak) to the sub's RMS rating at the same impedance. A sub rated 500W RMS at 2 ohms wants an amp that delivers roughly 400–600W RMS at 2 ohms. Reviewers and r/CarAV regulars repeatedly warn that manufacturer 'max power' or 'peak' numbers on budget brands are often two to five times the real RMS output verified on amp dynos, so cross-check with dyno tests before buying.