Thank you.

Oh shit now I understand

at 19 tiles, you need enough to buy the 20th tile and have as many academies as you are willing to wait - for me that was 5. Send out all the heroes on conquest at once and then make a purchase. The algorithm that calculates where the sea is will consider the unconquered tiles as part of the kingdom and spread out from the unconquered tiles as if they were conquered. This has already been brought to the attention of the developer a number of times, but since it's still part of the game, it's become a feature instead of a bug.

how tho