Brian,
A one quart bottle of Cat ELC Extender will treat a cooling system with a capacity from 7.5 to 12 gallons, so yes one bottle is all that you need.
David,
Cat publishes a time/mileage schedule for adding Cat Extender to Cat ELC Coolant, but the schedule is intended for engines used in commercial applications. Therefore, in my opinion, the best way to determine if it is time for coolant maintenance (extender or drain and refill) is to have an analysis ran on your coolant. If your engine was used in a commercial truck, coolant analysis is a yearly service.
Gerald