Is there a limit on how many steps LabAuthor's 'Undo' feature can roll back?

There is no known limit to the number of Undo or Redo actions that can be taken in a single session of LabAuthor.
However, it should be noted that once you Save the changes to an Experiment and close LabAuthor, the Undo and Redo options will be reset when you subsequently open the Experiment at a later date.
This information is valid for LabAuthor 4.5.2 or later.