How often are autistic children selective mutes?

Not really. Selective mutism implies normal speech and language in certain "safe" settings, and a voluntary resistance to speaking in other "frightening" settings or venues. Autistic children don't speak well because of their neurological deficiency. Some higher functioning autism patients may refuse to speak (in their normal way) in certain settings or with certain people, but this is not mutism.
Fairly often. I think it is actually pretty common, manyautistic kids have high anxiety and are inhibited in less familiar settings.