Brewing double batches of homebrew or brewing high gravity beers? You'll need lots of happy, healthy yeast to ensure the beer is fermented fast, thoroughly, & without off flavors. You could make a few smaller starters ... or you could skip the fuss and make one massive starter that will get the job done without a doubt.
Note: It is recommended that a 3-5 gallon stainless kettle is used to heat the flask in. Put two inches of water in the kettle and then place the full flask inside the kettle. Safer for both brewer and stove. Then whole kettle with flask can be brought to the sink and cooled while the flask remains inside the kettle.