As per the Excel Specifications and limits the maximum limit for undo in Excel 2010 is set to 100. In the previous version of Excel this was set to 16. The maximum limit value is fixed but you can increase or decrease this limit by modify registry settings.
Please Note:- Changing Registry Settings is not recommended and do this with at most caution while trying these changes.
To change the undo maximum limit, Open Windows Registry Editor and then navigate to the following path
HKEY_CURRENT_USER –> Software –> Microsoft –> Office –> 14.0->Excel->Options
Create a new DWORD value with the key name as UndoHistory
Then modify UndoHistory by setting the Base as Decimal and value as 16. Click the OK button to save the changes.
This would decrease the number of undo same as the previous of Excel. You can also increase the undo limit from 100 to 200 for Excel 2010 by changing the above created Windows Registry key.