I don’t know the answer to how many votes to indict. This is in state court so it depends on Missouri’s rules.
The following purports the following for MO.
Does the grand jury have to be unanimous?
No. The votes of nine members are required to find a “true bill” resulting in an
indictment being “handed up.” If the grand jury determines there is no probable
cause, “No True Bill” is found and the case is over.
http://news.stlpublicradio.org/post/whats-grand-jury-how-will-it-work-ferguson-case