The actual numbers- "Thirty-seven percent of the parents responded. Among the 359 respondents, 68% stated that all of their children had received at least 1 immunization, and 17% reported that some of their children had received at least 1 immunization. Only 14% of the parents reported that none of their children had received immunizations." from this - http://pediatrics.aappublications.org/content/early/2011/06/23/peds.2009-2599.abstract
To add to this:
It looks like 1/271 children have autism. http://www.opposingviews.com/i/myth-amish-don-t-have-autism
here was their method of collection to determine autism rates:
" Families were approached and asked to participate in a brief interview regarding their children. Two primary screening instruments were used: the Social Communication Questionnaire (SCQ) and the DSM-IV-TR Checklist (a tool created by the authors)...Children screening positive on either the SCQ or DSM-IV-TR Checklist were seen for a more comprehensive clinical evaluation by two licensed psychologists."
So…to sum up - the Amish do vaccinate less (quite a bit less - 32% completely unvaccinated, and who knows how many selective/delayed ) and they have less autism.
Of course, their lower autism rates might be due to factors other than vaccinating, but I think it is worth further study.