In twenty years, will there be fewer cars in use than today?

I don’t think so. Cars nowadays are the most important transport for medium-to-long distance travel. For now it’s hard to find a replacement. Although technology develops fast, scientists have only been able to find a more eco-friendly car, and no replacement. So it is hard to believe that a replacement will come in twenty years. Another important consideration is human population growth. This increases exponentially. There will be much more people on earth than now. As a result, demand for a better means of  transport will increase too. So, if none can be found, people will still buy the car and their numbers will grow. In conclusion, fewer cars in twenty years time than there are today seems unlikely.

Very well written!

In twenty years, there will there be fewer cars in use than today?

(但是你可以写这样：Will there be fewer cars in use twenty years from now?）

I don’t think so. Cars nowadays are the important transportation tool （"vehicles" 的字比 "tools"的字更好） for medium-to-long medium to long distance travels. For now it’s hard to find some substitute to replace it. Although the technology is developing , scientists just keep finding a more environmental -friendly （一个字） ways to use the cars instead of finding a new tool (vehicle) to replace it with. So it is hard to believe this kind of tool will emerge ("emerge" means there will be more cars! try using: "will be less") in twenty years later (可以说 "in twenty years" 还是 "twenty years later"). Another important fact ("factor") is the population growth on earth （可以说 "is population growth"). The population of humans  （之是对的，但是也可以说 "The human population") is increasing in exponential ways ("exponentially" 好听). There will be much more people on earth then than now. As the a results, the demand for the transportation tool ("cars") will increase too. So if there will be no new way for  mode of transportation, people will still buy the cars and the total number of cars will increase. In conclusion , the number of cars seems less likely (will unlikely be) fewer than today in twenty years later. （也可以说："In conclusion, it seems unlikely that there will be less cars in twenty years than there are today.")