The deepest realm levels most players can reasonably expect to get to is around 80-120. At that point many stat builds begin to lose effectiveness as the exponentially scaling monster levels become heavily problematic. Maximized builds with all Nethers / Nether Demons can get deeper, but I top out a few floors short of 220 without using spell spam.
Percentage builds have evaporated in the recent patches, but there are still a number of methods that completely ignore enemy stats to achieve victory, among them Paranormal Jellies and Raven Ritualists. These teams, barring specific counters (Bone Reaper + Mortarsman), can go infinitely deep.
There is no max realm level, nor even an effective one. One of the early patches stabilized the number engine well into the trillions, and I don’t think even Kejal’s tried to give the new one a heart attack yet.