TL;DR: RealPage Inc created a program that tells landlords what to charge for apartments, based on non-public pricing information from other landlords. It also encourages them to reduce supply. Over 20 million rental units are implicated by this anti-trust lawsuit.
The software covered 19.7 million rental units in 2020, but they've been growing rapidly, so it's well over 20 million rental units by this point – all of which now have tenants that might be eligible for class action damages.
https://news.bloomberglaw.com/esg/realpage-major-landlords-face-antitrust-lawsuit-over-rent-spike
https://www.propublica.org/article/yieldstar-rent-increase-realpage-rent