50,000 words is the goal of NaNoWriMo. It was chosen because it "is a difficult but doable goal, even for people with full-time jobs and children. The length makes it a short novel."  It breaks down to 1,667 words a day (1,613 words in 31-day months like July.) Many Wrimos abbreviate 50,000 as "50k" (k as in kilo, or thousand).
The 50,000-word goal came to be when NaNoWriMo founder Chris Baty chose the shortest novel on his shelf (rumored to be Brave New World), did a rough word count estimate, and came up with 50,000 words. While Brave New World is actually 64,575 words long, the 50,000 words goal stuck.
50,000 is a minimum, not a strict maximum; Wrimos are welcome to continue writing if they are so inclined or have not finished their novels - many do. However, they must make at least 50,000 on one novel to be verified as a winner in November.
As of 2013 Camp NaNoWriMo offers flexible word count goals for Wrimos who want to set a goal under 50,000 words or go above and beyond. Camp NaNoWriMo allows custom goals from 10,000 words to 1,000,000 words. This allows more flexibility for Wrimos writing scripts, editing, or pursuing other projects.
Some critics of Nanowrimo claim that the goal of Nanowrimo is too short to be a real novel, stating that most published novels are around 70,000 words. However, there are many novels, such as The Great Gatsby, which are around 50,000 words in length. As well, many novels expand in length during the editing process, and the 50,000 words written in November is just the beginning.