tinymce-docs-4x/_includes/configuration/custom-undo-redo-levels.md

custom_undo_redo_levels

This option should contain the number of undo levels to keep in memory. By default is set to use an unlimited number of undo levels. But this steals lots of memory, so for low end systems a value of 10 may be better.

Type: Number

Default Value: unlimited

Example:

tinymce.init({
  selector: "textarea",  // change this value according to your html
  custom_undo_redo_levels: 10
});