You've mentioned one of the big points of disagreement between Arminians and Calvinists regarding the interpretation of Romans 9–11 – are nations or individuals in view? Romans 9:6–24 is a crucial text in this regard. Another related point of contention is who exactly is Israel in various places, culminating in Romans 11:26 – a Jewish remnant only, the church as a whole, or a future mass conversion of Jewish people (see this overview of interpretations for more).
Turning to Romans 11:4 in particular, then, we would expect that Arminians would not see this as a reference to the individual election of seven thousand men, but to God's preserving those who are faithful to him. And this is what we found when we look at Arminian commentators like Adam Clarke, Jack Cottrell, and R. C. H. Lenski.
Clarke focuses on God's physical preservation of his people who faced persecution:
These [seven thousand] had continued faithful to God; but, because of Jezebel's persecution, they were obliged to conceal their attachment to the true religion; and God, in his providence, preserved them from her sanguinary rage. (Commentary)
Jack Cottrell focuses more on God's spiritual fellowship with this remnant, but unlike the Calvinist, he sees this as a result of their faithfulness to him. Quoting him at length:
God's statement, "I have reserved for myself," uses the verb [kataleipō], another "remnant" term (see v. 3). Its usual meaning is "to leave." Those with Calvinist leanings see an oblique reference to unconditional predestination in this world.
But this is not the point. Certainly this is an act of God regarding these men, but God's act is conditioned on the fact that they "have not bowed the knee to Baal." God is telling Elijah, "There are more than just you who have remained faithful. Indeed, I have identified and singled out from the great majority of Israelites a group of seven thousand true worshipers. I have separated them from the rest; in my sight they are a different group, a remnant. These are the ones I have kept in my saving grace and in close fellowship with myself."
They are the ones, God says, "I have reserved for myself." They are "his people" in a special, spiritual sense. In this spiritual sense only these seven thousand belonged to God; the rest were Baal's. This remnant alone was the true Israel of 9:6b. (Romans, 395–96)
But what about the use of the word "election" in the next verse (KJV)? R. C. H. Lenski argues that the sovereignty and grace described by Calvinists does not exist. Instead,
Grace makes an election in accord with its nature by taking for itself all those whom it wins for faith and for the acceptance of this gift of God's righteousness. (Interpretation of Romans 8–16, 683)
So here too, the "election" of these seven thousand is conditioned on their faithfulness – those won by grace are thereby "elected," whereas the Calvinist would reverse the order, with election leading to conversion and faithfulness.