A whale is a crypto term to describe a person who has a huge number of coin that can move or manipulate market. I guess if you have a 500BTC you can be called whale.
I think it is 100BTC enough
I think depend on marketcap of coin u invest... it marketcap is only 10k for total of this coin... 
U have 10 BTC. U will becam a whale in that marketYes it can but most probably i think hundred of btc are considered as a whale.
but I think if only thousand btc is not already called as a whale because for me, a whale have more than 10k btc in many wallet and that person can play with the price. I think there are people or person that have more than 10k btc and he don't spend much his btc and only waiting for next high price.