I had two ultrasounds with my last one and I'll probably end up with two this time, too, since I already had one for dating. My cycles were all messed up from BFing so I didn't know my due date for sure. They tried to just go by my LMP but I had a feeling that wasn't right. I ended up being two weeks earlier than I thought so I'm glad I got it because I don't want to be pressured for an induction two weeks early.
OP, I'm not sure what I would do in your situation. Six ultrasounds seems like a lot but it really depends on WHY you got them. I don't know if I missed it but I don't think you mentioned your reasons. Maybe in your case, it was worth it to get all of them.
I have family members who had autism and have done quite a bit of research into it. I really don't believe that ultrasounds cause autism, but I guess I can't guarantee anything for you so you do what makes you comfortable. I can tell you that as a teacher, I see many kids who are diagnosed on the spectrum, who as early as 15 years ago, would not have been diagnosed with Autism. I know it might not explain the entire dramatic increase but I really think it explains a lot of it. I have two cousins who were never diagnosed with anything but were always regarded as "different" from the other kids. One of them has since been diagnosed with Autism as an adult and the other one just continues to live his life and has quite a bit of social difficulty.