Jack Russell Terriers are defined as being more than 51% white.
How much more than 51% white is a matter of preference and genetics.
Color distribution on the coat of Jack Russell terriers is not random.
In fact, the creep of whiteness and the addition of color tends to follow clear patterns. As the good people at doggenetics in the U.K. explain:
Whichever white pattern a dog has, its white will always follow the same rules of spread. White starts on the farthest "edges" of the dog - the tail tip, the tip of the muzzle, the paws and the tip of the breastbone. This is known as the "trim" pattern. From there it spreads to cover the muzzle and forehead, the front of the chest, the lower legs and more of the tail tip, creating irish spotting. Next it spreads round from the front to the back of the neck, and creeps up the legs and tail. On a piebald dog, only the head, back and tail base may still be colored. The back coloring is the next to go, followed by the tail base, then the face markings. The ears will always remain colored unless the dog has a very high amount of white. The ears are generally the last part of the dog to turn white.
This is exactly what happens with most Jack Russell Terriers.
One way to think of it is that the dog retains color best in the most important areas of its body - around its internal organs (body and tail base patches) and its brain (ears and face patches) - and loses color easiest from the parts farthest from these areas. In technical terms, pigment "migrates" to different parts of the body during the development of the embryo, and the S gene determines how far the pigment migrates.
Folks that want to read a more detailed explanation of the alphabet soup of alleles implicated in coat color and spotting patterns can read this very good article from Holly Steel.
For the rest of us, the bottom line is that pure white Jack Russells come with a caution.
The extreme white pattern consists of a completely or predominantly white dog with just small amounts of colour on its head and sometimes base of tail.... Extreme white can occasionally cause problems when it removes large amounts of pigment from the face and ears. The most common problem is deafness (due to lack of pigment in certain parts of the inner ear, which prevents it from functioning properly)....
And, of course, deafness is a problem in extreme white Jack Russell Terriers, which is why I am always troubled when I see entire kennels full of pure white dogs.