In theory unlimited. You can’t retrieve former addresses used. The wallet however has the master key and accumulates all.
–CORRECTION-- apearantly you can but a bit of a trick: https://chiaforum.com/t/check-if-receive-address-is-tied-to-wallet/11102/4
But I’m pretty sure those reward addresses are not on your wallet, otherwise the .25 farmer rewards would have shown up and there would not be a warning about no keys for those addresses… Still, I was wrong about no way to retrieve former addresses.
–END CORRECTION–
If you never changed the Farmer and Pool reward addresses in Farm section they should still be the first address as in chia keys show, but then no idea why it’s complaining about no keys…