Too many functions used in MS Excel?

I have Excel sheets with more than twelve millions of formulas; I don't think there is any limit.

Your problem is probably that the cell format is set to 'Text', and whatever you enter is treated as text. Change the format to 'General', and edit the content afterwards (add an x and remove it), and it should work. Alternatively, you can copy a working formula into the field (which copies the format too), and edit it as needed.