I think that one currency will be used throughout the world as long as the coin is able to develop well and can survive onwards instead of using lots of crypto coins but the results are less effective, and we must be selective in looking for these coins, don't choose the wrong amount of coins this
If you compare with modern means of payment, it is obvious that there will be about 5-10, and maybe more coins, because there is more than one quality coins, but too many coins will interfere with each other in the development and just do not need