The simple answer is…there isn’t a max height. With that being said, there isn’t a reason to build a mile-high retaining wall when skyscrapers have more purpose to build rather than such a high earth structure.
As walls get taller, reinforcement is required to keep it standing. That reinforcement can be geogrid, anchors, or No-Fines concrete. When the wall cannot stay vertical by just using the blocks, then reinforcement is used to achieve a taller height. Without a limit to how high a retaining wall can be built, cost to benefit analysis will dictate how high you want to build your wall.