The number of words in an average novel can be anywhere from 70,000 to 120,000. Genre plays a role - fantasy and historical novels might be longer, while some contemporary fiction could be shorter. Also, some authors are more concise, while others are more verbose.
On average, a novel has approximately 90,000 words. However, this is just a rough estimate. Different publishers and literary circles might have slightly different standards. Some popular novels might have as few as 50,000 words or as many as 150,000.