Whale should have enough bitcoin and altcoin to change the trend and price by himself. For bitcoin it is very hard they must have maybe more than 100 million dollars of bitcoins, but it is easier to be an altcoin whale. if they can start collecting from deep price and collects enough (~5% of total circulating supply) of it and also have some bitcoin to pump the price.
They are people who have a high degree of belongings by their side,talking about cryptocurrencies,in other words,they have a big number of these,they can easily manipulate the market sometimes,sometimes they just accumulate and accumulate and other times they only release a lot of coins that they have causing marked changes in prices almost always are very visible when one or several whales are in action because the price moves severely or place great offers of purchase or sale.