As long as you get enough of them and you have enough time to cast them all, the use of 9th level spell scrolls will allow you to cast many 9th level spells without a long rest.
The DMG has rules for awarding epic bonuses (starting on page 231) that include:
Blessing of high magic
You gain a level 9 spell slot, as long as you already have one.
Breath of spell
You can cast any spell you know or have prepared without spending a spell slot. Once you do, you will not be able to use this blessing again until a long break is over.
Any of these blessings would allow him to cast Foresight (or another level 9 spell) twice before taking a long rest.
Monsters do not have the same restrictions as player characters and may also have more than one 9th level spell slot. An example of this in published adventures is (spoilers):
Acererak from the tomb of annihilation
1. Suggested by NautArch