There are no definite amounts for you to be considered a whale but one thing is for sure, you have to own a substantial amount of Bitcoin so as to affect the Bitcoin market when you decide to sell your Bitcoins or when you are eager to buy a huge share in the availability of Bitcoins in circulation.
Whales, as used in this business, can either be people, partnership, corporation or any entity, that owns a substantial amount of Bitcoin. Generally, they are into Bitcoin because they would like to influence the market when they buy and sell for their own benefit. Through their acts, the average investors may greatly benefit from it or may substantially lose a huge amount of sum of money.
It is quite simple. If you have such big amount that with your one move, either sell or buy, market can be shaken, you are a whale.
There is no limit or criteria for a trader to become whale. This can be a single person or group of traders who have same joint goal and working at same thing. These whales have such a strong impact on international market that they can destroy much of it at once