According to this post and comparing the Steem supply distribution across all accounts, you're a whale if you control at least the 0,253% of the allocation of the new tokens created.

According to the figures of 10 days ago, 4398,77 LEO tokens in stake are required. This amount will go up over time obviously as the token distributes across accounts.

https://www.steemleo.com/tribe/@holger80/what-is-the-amount-of-staked-tokens-to-became-a-minnow-dolphin-orca-or-whale-for-a-scot-tribe-2019-07-09

Regards!