Most quit in the first week or so, deciding that life on the trail is not for them. Injuries or illness forces another large percentage off the walk, and that can happen at any point along the trail. Some of the rest do finish, just not in the year they start the hike.
Most begin the hike on their own, without a hiking partner. Often, this is because the logistics of two acquaintances having the time and means in the same year to make the hike is challenging. Still a sizeable minority do plan a hike and finish with a partner. Parent-child combinations are more common than you might expect, though friend-friend, both of the same and opposite sex, are the most common.
Typically, people who start the hike at about the same time continue to see each other throughout their long walk and often become friends, joining up to hike with each other for a day or a week, off and on throughout the hike. Through hikers sign trail registers along the way, and this is how they know where and when a compatriot passed the register. Cell phones are common, of course, but coverage can be spotty in the mountains. The trail register is used as much as the modern technology.
Shelters abound along the trail, notorious for their mice. Even people who hike together will likely not always walk together. Sometimes “together” simply means agreeing to camp at the same spot each night. Differences in partner height can make walking together uncomfortable. A tall person takes a lot fewer steps per mile than a short one. Trying to adjust your own pace to someone else’s slower or faster one is a lot more tiring than walking at the pace that’s comfortable for you.
It takes 4-5 months, on average, to hike the entire trail. Hikers typically cover 12-16 miles a day, once they get their trail legs underneath them. You will not be surprised to learn that the distance hikers cover at the beginning of the hike is much shorter at the start of the trail than by its end. For the first 2-3 weeks, hiking distances are often in the 8-10 mile range. The best way to train for the long hike is to walk long distances with the same pack you will carry during your walk, and virtually no one has the time to do that properly.
If a hiker makes it to the halfway point, as the two I met on Sunday already had, they will almost certainly finish the hike. After 1000 miles or so, the body is well-used to the daily physical stress of the walk, and a hiker has already proved they have the mental attitude they need to finish.
I rather envied the two I met--healthy and strong, able to put aside work or family life, unwilling to sit idly by while life races past--all to pursue a dream that called to them. It's easy to stay in the main stream of life, to follow the kind of life that "everyone" lives. It takes a different sort of person to step away and do something different. I take my hat off to you. Good luck to both and safe travels.