Is there a hints cap given by hint Toad? If so, what is it?
TL;DR: There is no limit on the hints, but Hint Toad can only sell you hints for moons that you can actually get. If he doesn't sell you any hints, then you cannot get the moons at that time.
Also, he won't sell hints to moons whose locations are already known.
Hint Toad will give you hints for any moons that are available on the current map.
This means that you will never get the locations for the following moons:
- hint art moons (the hint art itself will be shown by Hint Toad)
- boss/progress moons moons (as Hint Toad only appears after you completed the Kingdom's story)
- moons that are currently not available (e.g. in broken Cap Kingdom)
Toadette moons, as they are linked to achievements
Moon rock moons, as they will only appear after you defeated Bowser, and will get marked automatically on your map, so no need for Hint Toad to act there.
For example, the following moons are listed in the Cap Kingdom:
1. Frog-Jumping Above the Fog
2. Frog-Jumping from the Top Deck
3. Cap Kingdom Timer Challenge 1
4. Good Evening Captain Toad!
5. Shopping in Bonneton
6. Skimming the Poison Tide
7. Slipping Through the Poison Tide
8. Push-Block Peril
9. Hidden Among the Push-Blocks
10. Searching the Frog Pond
11. Secrets of the Frog Pond
12. The Forgotten Treasure
13. Taxi Flying Through Bonneton
14. Bonnetter Blockage
15. Cap Kingdom Regular Cup
16. Peach in the Cap Kingdom
17. Found with Cap Kingdom Art
You can only get moon 11-15 and 17 after Cap Kingdom has been restored, e.g. after you bested Bowser. Therefore, Hint Toad will only give you hints for moons 1-11 when you revisit Cap Kingdom before you defeated Bowser, as you cannot get any other moon at that time.
Furthermore, moon 16 (and all similar moons in the other worlds) won't trigger unless you talked to a key character beforehand, so Hint Toad won't show you that moon either, as it won't be available.
In order to get moon 16, Princess Peach needs to go on her world vacation tour. You have to talk to her in the castle after you defeated Bowser, otherwise you cannot get any of the "Peach in the <Kingdom>" moons.