hi.
As mentioned, there are the bladder tank and the float switch. Some water filter companies (e.g., SpectraPure) sell water level controller, and you can buy them or DIY. Some work by water pressure, some work by light sensor, while some work by sound.
The disadvantage of bladder tank is that the back pressure as the tank is filled make the RO less efficient. Permeate pump help a little, but it may be not worth it if you only use RO infrequently.