The default has been 16 since Excel 97 at least.

From Excel 97 help.......

Maximum number of undo levels 16 (default)

See Paul's post on how to increase this limit.

Gord Dibben Excel MVP

On Mon, 6 Dec 2004 06:09:05 -0800, Austrian Hannes
wrote:

The number of items that can be undone using 'undo buttons' appears to be
sixteen in Excel 2002. Can this number be increased?

I am sure in previous versions of Excel the number of undoes was much
higher, one hundred springs to mind.