Generally speaking, both Missouri and Illinois use 12-person juries. In Illinois, however, if the claim is for less than $50,000, a 6-person jury is used unless either side wants a 12-person jury. § 735 ILCS 5/2-1105.
In Illinois, the jury must reach a unanimous verdict. See IPI No. B45.01 Instruction on Use of Verdict Forms. In Missouri, only 9 of 12 jurors must agree in order to reach a verdict. See here and here.
In requiring a unanimous jury, Illinois is in the minority among states. According to statistics from a 1995 US government survey, thirty-three states do not require a unanimous verdict.