I thought there would be a limit too, but after some further investigation I found this:
"Keys can be bought in groups of 15, 35, 75, 200, or 450 keys, with a respective 40, 120, 400, 1,200, and 3,000 bonus Hearts of Ice given to the player upon purchase. There isn't a cap on how many bought keys and Hearts of Ice that a player can have at a time.
Players can spend an unlimited amount of money on keys. Players can only buy 20,000 keys in a single day. There is no monthly maximum when purchasing keys
."
Source: https://runescape.fandom.com/wiki/Treasure_Hunter
I guess the good promo will be over once I get this resolved :\
01-Jan-2019 17:59:35