When you use Excel's Undo feature, and you reach undo number 16, the first undo is replaced by the 17th, and so on. Also, as soon as you save your workbook, the Undo Stack is wiped out, and the Undo History is lost. This is because when you press Save, you're actually telling Excel that you are happy with the changes you made, so it decides for you that your Undo History is no longer required.
You might have discovered that at times having only your last 16 changes retained in the Undo Stack is not enough. Instead of living with this, you can change this by editing the registry, something that works only in Windows. To do this, begin by quitting Excel completely. Select Start » Run, and in the Open box type
Regedit.exe. Click OK. When regedit starts, expand the folder for HKEY_CURRENT_USER. Then expand the Software folder underneath it, then the Microsoft folder, the Office folder, and the 10.0 folder. (This last folder varies for different versions. 10.0 is for Excel 2002.) Expand the Excel folder, and finally, open the Options folder.
Select Edit » New » DWORD Value, enter the word
UndoHistory, and press Enter. Double-click the UndoHistory you just created, and enter a value greater than 16 and less than 100.
Hopefully, 100 undos will be enough for even the most demanding users, though the issue with save clearing the undo stack persists.