We don't know how many housing units use Realpage pricing algorithms in 2024, but in 2017 when Realpage bought the tech, it was being used in 1.5 million rentals (https://www.realpage.com/news/realpage-to-acquire-lease-rent...) and there were 43 million rentals in the US, which means nationally they had less than 4% marketshare.
Obviously, cartel behavior (raising prices above market) does not work if you have 4% of the market. Perhaps their sales have gone up 20X in 7 years and they have enough market power for cartel behavior (doubtful)? Perhaps they have a low marketshare nationally, but they have a high marketshare in a few specific markets? Maybe, lets see what the DoJ says.