How often does encephalitis cause such damage that speech and physical therapy is needed?

Depends. It depends on the cause of encephalitis, its severity, location and timing of intervention. If the areas of the brain that are responsible for speech and limb movements are involved, there will be deficits that will require speech and physical therapy started.
See below. Encephalitis is a general term for an infection in the brain. There are both viral and bacterial causes as well as other causes. The symptoms can include, paralysis, loss of sensation, impairment of speech and cognitive problems. It is a very serious condition and usually extensive rehabilitation is required.