All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
What you mentioned that a person subtracts what he needs for his expenses and then pays Zakah on the remaining money if it reaches the Nisaab (the minimum amount liable for Zakah), then if by this you mean that he subtracts what he expects to spend on himself and his family during the lunar year and then pays Zakah on what remains, then this is the opinion of some scholars however, this is not the most preponderant opinion. On the contrary, most scholars are of the view that it is an obligation on a person to pay Zakah on everything that he possesses at hand when a whole lunar year elapses after subtracting the debts which he owes.
As regards your question about owning more than one car, then owning two or more cars is not forbidden as Allaah Says (what means): {Say, “Who has forbidden the adornment of [i.e. from] Allaah which He has produced for His servants and the good [lawful] things of provision?”}[Quran 7:32], especially if the person needs the second car, but it is more appropriate for a person to own only what is sufficient for him and he should not be too keen on owing so many kinds of permissible things. This is in regard to the ruling of owning more than one car.
With regard to paying Zakah on cars, then in principle there is no Zakah on cars, because there is no Zakah on the wealth that one owns for acquisition and personal use. However, if these cars or some of it are prepared for trade, then it is an obligation to pay the Zakah on the cars that are prepared for trade if it meets the conditions of Zakah on tradable items, which is that one should own it and he should intend when owning it that it is for trade. Therefore, if the tradable items meet the conditions of Zakah, it should be valued at the beginning of the year, and then Zakah should be paid on it, which is 2.5% of its amount.
As regards the meaning of Zakah on money, then you should know that it is an obligation on a Muslim to pay Zakah on the money which he possesses and other tradable items if it reaches the Nisaab and a whole lunar year elapses on it. Therefore, you are obliged to evaluate the cars and textiles and other items that you trade in, and then you should pay the Zakah on it at the beginning of every lunar year, which is 2.5% of its value –whatever its value is –and what remains of it for a whole lunar year or two whole lunar years without it being sold, then you are obliged to pay Zakah on it as well because it is a tradable item.
Finally, saying that you do not think to have the cash money in order to pay the Zakah that is due, then this is most unlikely because Zakah is only 2.5% of the values of the goods, and if we presume that you do not have cash money, then it remains as a debt on yourself until you are able to pay Zakah on it. For more benefit, please refer to Fatwa 90055.
Allaah Knows best.