If God created Adam (the Hebrew race for the line of Jesus) on Day 6, did he use an 8th day to make the rest of the human races?
Your suggestion that God created "races" on an eighth day of Creation is not found in Scripture, and in fact, it contradicts Scripture.
First, the Bible is clear that there were only six days of Creation. The Bible never records an "eighth" day of Creation, and it says explicitly that God rested from all His work of creation on the seventh day and He did not resume His creative work thereafter. This is the meaning of the seventh day Sabbath...God in perpetual rest:
Gen. 2:1 Thus the heavens and the earth were completed, and all their hosts.
Gen. 2:2 By the seventh day God completed His work which He had done, and He rested on the seventh day from all His work which He had done.
Gen. 2:3 Then God blessed the seventh day and sanctified it, because in it He rested from all His work which God had created and made.
Heb. 4:3 For we who have believed enter that rest, just as He has said,
“AS I SWORE IN MY WRATH,
THEY SHALL NOT ENTER MY REST,”
although His works were finished from the foundation of the world.
Heb. 4:4 For He has said somewhere concerning the seventh day: “AND GOD RESTED ON THE SEVENTH DAY FROM ALL HIS WORKS”;
Heb. 4:5 and again in this passage, “THEY SHALL NOT ENTER MY REST.”
Secondly, the concept of "race" is a human invention and not something established by God. In fact, there is no Biblical basis for designating different "races" of humanity, since differences in physical features from one human to another are completely arbitrary and meaningless. On the contrary, the Bible teaches all human beings are of the same race: