Most homebrewers brew 5 gallons (19 L) at a time. The vast majority of homebrew recipes are written for this size and 5-gallon (19-L) batches are the de facto standard for homebrewing. However, there’s nothing magical about 5 gallons (19 L). You can brew more or less beer if it fits your situation.
The 5-gallon (19-L) batch size is popular because a variety of items can be repurposed for brewing batches of this size. Five to 6.5 gallon (19–25 L) carboys and 5.0-gallon (19-L) soda kegs are the two most obvious examples. Also, it produces an amount of beer — a little over two standard cases (of twenty-four 12-fluid-oz. (355 mL) bottles) — that most homebrewers feel is worth the effort. However, depending on how often you brew, the space you can dedicate to brewing and storing beer, and how much beer you want to have on hand at any given time, you can scale your homebrewing up or down.