Need for Speed Unbound

This achievement requires you to own 10 vehicles in your story mode garage.

This can be earned quite easily as you get a ton of free vehicles from completing qualifiers, competing in races in story mode, and completing all of the activities and collectibles. In fact, you'd earn enough free vehicles to unlock this without having to purchase another one if you're going for the 100% completion. Below are the vehicles you can get for free that are tied to achievements:

Qualifier/The Grand Cars:
-2015 Land Rover Defender 110 Double Cab Pickup
-2008 Ford Crown Victoria
-2017 Nissan GT-R Premium Edition
-Your starter car from the prologue of story mode (mine was the 1988 Lamborghini Countach 25th Anniversary)

Activity/Challenge Reward Cars:
-1981 BMW M1
-2016 BMW M4 GTS
-2015 Porche Cayman GT4
-2018 McLaren 600LT
-2015 Nissan 370Z Nismo
-2016 Ford Focus RS
-2015 Ferrari 488 GTB
-2017 Lamborghini Aventador S Roadster

There are also vehicles you can win throughout story mode from specific races, however I believe these are optional and potentially missable. Here are the ones I won for reference (I believe this is all but one of them):
-2009 Lamborghini Murcielago LP 670-4 SV
-2006 Lotus Exige S
-1990 Ford Mustang Foxbody
-1970 Plymouth Barracuda
-2016 Alfa Romeo Giulia Quadrofoglio
-2007 Mitsubishi Lancer Evolution IX
-2018 McLaren 570S Spider
-2019 Dodge Charger SRT Hellcat
-1964 Aston Martin DB4

You can also earn A$AP Rocky's custom 1988 Mercedes-Benz 190E 2.5-16 from completing his mini quest line within story mode.

So as you can see, you can earn up to 22 cars for free (not including the one you choose to start chapter 1 with after the prologue), and 12 of these are tied to achievements required to 100% the game. However, if you feel the need for some reason to spend your hard earned money to get this achievement faster, you can buy 9 1965 Chevrolet C10 Stepside Pickups or 1963 Volkswagen Beetles for $162,000 when you unlock them (which should be right away or fairly early on.

However you choose to go about it, as soon as you get 10 cars in your garage, you'll unlock your achievement.