During amniocentesis do they stick the needle in the sac more than once?

Not usually: When having an amniocentesis we use ultrasound to help us guide the needle into the correct area. Of course you have to consider experience when having an amniocentesis done. A physician with experience usually can perform an amniocentesis with one puncture.

Just once: Once is all that is needed, and it's usually done with ultrasound guidance. Occasionally a second puncture may be needed if multiple sacs (twins, triplets, etc) but even then it can usually be avoided.