Selective mutism is more common than autism.

Studies in California and Israel have shown that seven out of every 1000 children has selective mutism. Selective mutism (SM) is a condition in which children are unable to speak in certain situations (like school), even though they know how to speak. It’s commonly associated with anxiety, and is often misdiagnosed as autism, mental retardation, or oppositional-defiant disorder. SM has a prevalence rate of 7.1 per 1000, making it more common than autism (0.5 per 1000), major depressive disorder (0.4-3.0 per 1000), Tourette’s (0.5 per 1000), and obsessive-compulsive disorder (0.5-1.0 per 1000).
(Sources: 1, 2)