They can print as much or as little money as they want. But if you can just make unlimited money, the value of that money goes down (it is called inflation). Same as old school baseball trading cards. If you have some rare card it’s worth a lot more. If everyone has a copy of that card, now it’s not rare and it is worth less.
Hypothetically they actually do as they are the one and only group that prints the money.
The problem is that printing money ad infinitum depreciates the value of that money (i.e. it causes inflation). Money is, fundamentally, a store of value, and thus heavily inflating money makes it kind of terrible at storing value over time. Thus, it doesn’t matter if you print infinite money if no one wants said money.
A government can always increase the amount of money it has in circulation either physically or just on paper. The issue is they can’t increase the amount of goods available to purchase with that money.
As more money becomes available and the number of things it can purchase remains the same, then inflation is the result. Goods will cost more because of basic supply and demand.
Take a look at Venezuela for an example of how this pans out. When the money is worth less than the paper it’s printed on.
Many governments ruined their money by printing too much of it. germany after WW1, Argentina after WW2, Zimbabwe recently. Others explained that part well enough.
One special case however would be the USA. Most of the dollars in circulation aren’t in the USA. The money is so stable, over 70% of all international transactions use Dollars as one of the two currencies involved, or straight dollar to dollar. Until recently, the USD was the only currency accepted at all to buy crude oil. Even Soviet Russia sold their oil for USD.
There is so much economic activity involving the dollar that the USA can, and definitely does print more money than everyone else without provoking hyperinflation. The flip side of that is that if the USA wants to keep its dollar as THE main currency that everyone else pegs their currency to, they can’t go stupid and print absurd sums either. If they lose the statuys as the world’s reserve currency, many bad things would happen economically, so the FED is fairly careful about how much money they print.
You might be asking about Modern Monetary Theory without realizing it. This policy posits that a government could have infinite money if it wanted. However, there are many other things to consider like impact on inflation, taxes, employment, etc.
Have a listen to this NPR Planet Money podcast on [Modern Monetary Theory](https://www.npr.org/sections/money/2018/09/26/651948323/episode-866-modern-monetary-theory). Or read this [article](https://www.investopedia.com/modern-monetary-theory-mmt-4588060) on Modern Monetary Theory which explains a bit further.
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