In theory, the amount of money in the world should reflect, accurately, the value of all the stuff we own and do. By “we,” I mean humans. The purpose of money is to allow folks to exchange value without having to barter shit. Barter is a horribly inefficient way of doing things, and the world’s economies would collapse if we have to use it. Anyone who suggests barter, probably knows very little about economics. It would most certainly make things far more complicated.
Money is simple. It represents stuff we find valuable. There is not actually the amount of paper denominations necessary to reflect all the goods and services in the world, but that isn’t necessary. In fact, we don’t need paper at all. We just need numbers in a computer. So long as we are all in happy agreement that the numbers in the computer are fair assessments of the value of things, we’re golden.
Money is just numbers and the number applied to the value of something is arbitrary and constantly changing due to inflation, deflation, and exchange rates. You could say that the value of goods and services in the world is 100. Or you could say it is eleventy trillion. You could say it was 90 trillion dollars or 100 trillion pounds or 80 trillion Euros or 200 trillion renmin. It doesn’t matter. It could be any of those things, so long as everyone else agrees that’s what it is.
The answer to your question is “enough.” There is enough money in the world to keep the economies moving. That is the point of money: to make sure that people believe money reflects proper value. As long as that is true, people will buy and sell stuff. Everyone will be happy.
When there is inflation, people believe that there is slightly too much money in the world. More than there is stuff. So it becomes less valuable. Deflation is the opposite. It says there is not enough money for all the stuff, so the money becomes more valuable.
Inflation is considered good. Hyperinflation, though, is bad. Because that says the powers that can print money are printing way too much.
If you want, you can come up with a number. You’ll have to standardize on some currency, since all different currencies value things differently. Then you add up all the stuff and services int he world. Of course, that number will be meaningless. It’s the stuff that counts, not the number you put on stuff.