Orcad variables in text
I'm not sure of the general case.  Your specific example, however, is accomplished this way:

By default, each of your new schematic sheets will have a title block in the lower-right corner, called TitleBlock0 in the design hierarchy.  There will be text in the title block of each page saying "Sheet 0 of 0".

Although you can double-click on each of these to change the values, these fields are automatically calculated and updated when you Annotate the design (Tools -> Annotate).  Make sure you select your .dsn file in the Project tab, or Annotate will be grayed out.

Within the Annotate dialog, choose either "Incremental reference update" or "Unconditional reference update", depending on your needs.  The Help button on this screen is actually helpful :)

Also, there is an easy(ish) way to manually edit other Title Block fields.  In the Project tab, right-click on your .dsn file, and choose Edit Object Properties.  Now, in the bottom of the screen, there will be a tab labeled Title Blocks.  This gives a spreadsheet containing each sheet's title block, and you can copy/paste values down the columns.

Good luck!