Well, Human Wizard, lvl 20, int 18 at 1st level.
23 with level ups.
+5 Inherent.
+6 Enhancement.
That's 34 Int.
Base DC for a 9th level spell: 10+9+2 (spell focus and greater spell focus)+12= 33.
Some examples:
Old Red Dragon, CR20:
Fort: +23, Will +21.
Balor, CR20:
Fort: +22, Will +19.
Pit...