Forza Horizon 2

For this achievement you simply need to have 50 cars in your garage at one time, which will be quite easy to achieve and will come along the way while going for
Now for the 50 cars you are allowed to have duplicates of cars and any DLC cars you purchase for 0cr also count so I suggest you download the free launch bonus car pack here and the Mobil 1 car pack here, downloading these two packs will give you 9 cars for free.

You can either let this come through natural progression since there is a chance to get a free car from a wheelspin or a quicker way would be to download the same car over and over, the cheapest car in the game is the Volkswagen Rabbit GTI which costs a mere 5,000cr.