Currently I am sitting on 20 barrels and I will be adding about half a dozen more.
The Medieval Period was definitely one where the barrel was king. Or I should say, cask. The barrel was a specific kind of cask with a specific measure. The following graphic will give you a sense of the measures. Eventually, the barrel is defined as 36 imperial gallons. Smaller casks also existed, the kilderkin was ~18 gallons, the firkin at ~ 9 gallons, and the pin at 4.5 gallons.
Now, casks are mostly defined as liters. The “pin” is about 15 liters.
Modernly, hobby brewers can get barrels from 1 to 20 liters. Professional distilleries and breweries can get them much larger, up to 200 liters.
Before use, the cask needs to be cleaned out. Fill with clean water and rinse until it always runs clear. Most likely, there is some loose wood to be removed. This will not hurt your liquor but it might clog your spigot.
Most casks are toasted to varying degree. Toasting converts some of the wood chemicals to vanillin and removes some of the more harsh resins. It also imparts a darker color to the liquor. It is why whiskey and bourbon are brown in color. But after a few agings, the chemicals and color will fade. The cask is considered “neutral” at that point. It is fine for storage but not for imparting flavor to the liquor.
Because smaller casks have a higher surface area to volume ratio, their equivalent “year” is less than the standard 200 liter (53 gallon barrel).
For the most part, I am aging in 2L barrels. My “year” is 4.5 times as fast as the normal year. This is fine for a hobbyist but not for someone who wants to produce enough to sell.