The atmosphere has a mass of 5.15×10^18 kg. The concentration of CO2 is about 400 parts per million. That means there is 2.06x10^15 kg[*], or 2.06 trillion tonnes, of CO2 in the atmosphere. That works out to about 294 tonnes for every man, woman, and child in the world. There are also vast amounts dissolved in the oceans.
About 2.4 kg of CO2 is produced per litre of motor fuel burned; hence synthesizing motor fuel from CO2 requires about 2.4 kg of CO2 per litre. That means that the 2.06x10^15 kg of CO2 present in the atmosphere could generate over 8x10^14 litres of motor fuel, or more than ten thousand litres for every man, woman, and child in the world.
So around now, if you are a US driver, you are probably thinking that you do consume on the order of 500 gallons, or 2000 litres, of motor fuel per year, and you will note that there are other vehicles besides personal motor cars to be considered - trucks, planes, ships, etc.
But it seems to me you are utterly ignoring the overriding point. It is a giant closed system! Every kilogram of CO2 you process into fuel gets burned, and every single kg of CO2 you release from burning the fuel goes back into the atmosphere. And the overall loop is very nearly lossless. Sure, some very small fraction of the carbon liberated by combustion gets turned into CO or C particulates instead of CO2, but with modern pollution controls that fraction is very slight.
There are enormous logistical challenges to using the technique at full scale (including where to get the staggering amount of energy to run the synthesis), but running the atmosphere short of CO2 is not one of them.
[*] I spent a fair amount of time researching and could not readily determine whether the oft-quoted figure of 400 ppm is by volume or by mass. My math assumes that it is by mass. That actually leads to lower figures (pessimistic to my point) than if it is by volume, as it probably is. This is because CO2 is substantially higher density than air.