The History States control of the number of undos that are available.
For Photoshop Elements
The History panel: F10 or Window > History
This will let you jump to any recent state of the image created during the current work session.
The number of undos that are available is what you set it to be or you could have up to 1,000 undos. A word of caution when you start to undo your work you may be undoing something that you actually want. This is why it is nice to work with layers. You can just delet or hide a layer and continue on.
By increasing the number of History States you will also increase the amount of RAM Photoshop uses. When you assigning more RAM to manage the History will give you less memory for Photshop to perform other operations and will reduce the performance of Photoshop. If you find that you have a high History State your can lower. This may speed up a slow performaning experience you are having with Photoshop.
The default of the History panel is 50 previous states. By lowering this your may find a difference in performance.