Asphalt Legends Unite

As of the time of writing this guide, there are 51 cars available in the game, and you need to unlock 30. Unfortunately, many of the cars (especially many A and S class ones) are unavailable unless there is a special event to win blueprints for them, so I wouldn't even bother with them.

Speaking of blueprints, cars are unlocked by obtaining a set number of blueprints for that car. This could be only 5-10 for the first cars, or as many as 60 for some other cars. There are a few different ways you'll be getting car blueprints, so you'll need to utilize all of the game modes and features to unlock all of the cars unless you get really lucky. Here's where you should focus your efforts for the various cars:

Career Mode
Mitsubishi Lancer Evolution
BMW Z4 LCI
Chevrolet Camaro LT (also Daily Loot)
Nissan 370Z Nismo (also Daily Loot)
Dodge Challenger (also Daily Loot)
DS E-Tense
Dodge Challenger 392 Hemi Scat Pack
BMW 3.0 CSL Hommage
Lotus Evora Sport 410
Mercedes-AMG GT S
Aston Martin DB11
Ferrari LaFerrari
Icona Vulcano Titanium

Daily Goals
Lamborghini Asterion

Daily Loot Event
Porsche 911 Carrera GTS
Aston Martin Vulcan
Lamborghini Centenario

Daily Events/Legends Store
Volkswagen XL Sports Concept
Lotus Elite Sprint 220
Chevrolet Camaro ZL1 50th Edition
Dodge Viper ACR
Pininfarina H2
Jaguar F-Type SVR
Exotic Rides W70
Ford GT
Dodge SRT Viper GTS
Ferrari FXX-K

Multiplayer or My Club Rewards
BMW M4 GTS (silver pack)
Ford Shelby (gold pack)
McLaren P1 (silver or gold pack)
W Motors Fenyr Supersport

Special Events and Special Card Packs ONLY
Acura NSX 2017
Rezvani Beast X
Chevrolet Corvette Grand Sport
Ferrari J50
Lamborghini Aventador SV
VLF Force 1 V10
Porsche 918 Spyder
Pagani Huayra BC
Lamborghini Egoista
Trion Nemesis
Lamborghini Terzo Millenio
Koenigsegg Regera
Bugatti Chiron

While this list isn't entirely exhaustive, it's meant to show where to focus your efforts for each car. Many cars have blueprints in career mode but won't be unlocked there. You'll get some blueprints for both Dodge Viper cars, for example, but nowhere near enough to unlock them. All of the cars on the last list are ones you can expect to never unlock unless you play the game for maybe 6 months or so real time, or spend tons of real money, or just get really luck with the events that occur during your time with this game. To put it in perspective, you can buy an S-class car when it's "on sale" at an 80% discount for $100. Yes, $100 for a single car. So if you have no problem spending literally hundreds of dollars, you can unlock cars relatively quickly. If you don't want to spend any money, expect to put in somewhere around 150 hours over the course of at least 2 but possibly 3 real months.

The reason for the long real time is because you can't change the time and date on your device to speed anything up, so you're forced to wait real time for the daily goals to refresh to work on your Lamborghini Asterion, for example. I highly recommend reading the two linked posts in the roadmap for time and resource management. Their goal is to get you new and starred up cars as quickly and efficiently as possible. The more you stay on top of each game mode, the faster you'll unlock cars. Additionally, absolutely make certain to complete the daily goals each day to get Lamborghini Asterion blueprints to unlock that B class car, and participate in the daily loot event every single day to make progress towards unlocking the Aston Martin Vulcan (A class) and then the Lamborghini Centenario (S class). Those three cars will get you through a huge amount of the career mode seasons and will be extremely useful in earning coins and tokens in other daily events, as well as competing in multiplayer.

This will be your last achievement almost assuredly, so once you unlock your 30th car you'll finally obtain this long term achievement.