Upvote:5
Book directly with the car rental company and not with an aggregator.
Aggregators can have murky contracts with the rental car places and roles & responsibilities are not always clear. If you book with a smaller outfit or someone you've never heard off, check on-line reviews before you book.
While nothing is ever 100% guaranteed it would be exceedingly rare for a reputable car rental company to not give you a car at all. They may not have the class of car that you have booked but that typically gets you a free upgrade to the next available bigger class, but they will go out of their way to find you a vehicle.
I've rented dozens if not 100s of cars this way and never did NOT get a car. Maybe not the car I wanted or booked, but there was always a car.