Maybe. There are only four Crew Dragons. One’s attached to the station right now, and one’s had its docking equipment removed in preparation for the spacewalk on the Polaris Dawn mission. That leaves two. I’m sure one’s already in prep for the next regularly scheduled crew rotation. A rescue mission would mean either leaving two of the astronauts scheduled to fly that flight on the ground to leave open seats for the Starliner crew, or a special mission using the last Dragon.