If you do not want to use sheet sets (which I highly recommend, as it makes project pkg management simpler) create an attribute for the total sheet count.
ROUGH MANUAL OPTION 🙂
Edit the attribute inside the border block >> right click inside the edit row of the attribute editor dialog and slect insert filed. You can link this to any of the current document properties which you can edit the in or out of autcad >> still a manual process unless you use something like hurricane script tool
SIMPLER OPTION
Create an Xref called Total Sheet Count >> base ref 0,0
have a single piece of text in the file that has the total sheet count that you edit as needed.
Insert it as an Xref in each sheet >> use this technique for revision and release date too if applicable.
Sheet set manager is much simpler you get to edit a sheet set property which updates on every sheet.
The only other way to do the auto math version you wish to implement would have to happen through a script. Again I would suggest Hurricane as it makes script writing simple. This technique would allow batch processing , smack the go button and go to lunch it is a LARGE project.
Best Regard
-Tim C.
Best Regards
Tim