They obviously didn't exist, but Abraham at one time lived very close with Lot, and had some idea about Lot's household. His concern in praying was for the household of Lot. I believe he prayed until he thought Lot would be spared.Who would those ten righteous people in Lot's household be? Since Lot and two of
his daughters were the only ones who ultimately escaped the destruction of Sodom.
As far as who in particular, I heard a preacher give a list of 10 possibilities from Lot's household that seemed plausible, but don't recall it any longer.
The alternative is to believe that Abraham didn't really care if Lot was saved or not; that laboring in prayer with God was not evidence of his love for Lot, and his great desire for him to be spared. You have loved one's you pray for in the same way. Do you pray that they might be saved, or that they indeed do become saved?
I can't prove from scripture that Abraham prayed as I have suggested, but I believe he did.