When you "invest" in a fiat currency, willingly or unwillingly, you're holding money with serial numbers. The money printers promise you that they're difficult to counterfeit so you don't have to worry about duplicate serial numbers. (Yet they're counterfeited all the time, mostly by hostile nations but also by the issuing nation when sending billions to foreign terrorists such as Iran).
The money printers do NOT ensure they won't print more money with new serial numbers that devalues your fiat money. In fact, they're printing more all the time. The value of your fiat money is what the printers say it is by adding or removing some from circulation.
Cryptocurrencies on the other hand promise you a distinct serial number that's difficult to counterfeit since each transaction tracks the serial number (unlike transactions with bills). Cryptocurrencies offer a fixed number of digital "coins" or at least a schedule of how long it takes to "mine" new ones and how many might be added. The market then determines the value of each "coin".
But bitcoin is constantly being manufactured. There are an infinite number of bitcoin serial numbers