Those he decides to have mercy on.
Romans 9:15 KJV
15 For he saith to Moses, I will have mercy on whom I will have mercy , and I will have compassion on whom I will have compassion .
The whosoevers are defined in Romans 9 before you get to 10.
Yer tryin' to put the cart ahead of the horse.
Jesus is the savior and reconciler of the whole world, 2 Corinthians 5:19.
Salvation is a free gift from God that has been provided for all by Jesus Christ, Ephesians 2:8.
God is not willing that any should perish, 2 Peter 3:9.