Not sure what you mean, but an Uno has a 10-bit A/D which means 2^10= 1024 steps.
So if you design your voltmeter (resistor divider and code) right, then you only have 1024 display steps.
A "9.999" display would need 10000 steps. That's 10 times more than what you have.
A "50 volt" voltmeter would only have 1024/50= ~20 steps per volt.
ONE decimal place (10 steps per volt) would be ok, two or more decimal places would be impossible.
Leo..