I'd like to render an object only at Frame 20. The common approach seems to be set 3 keyframes at Frame 19, 20, and 21. It's not too bad, but I really wonder if I can just use the current frame number as driver input. If it is possible a single driver is enough to express "only render at Frame 20".
There are many ways to do this. One of the easiest is to use the frame number as an input to control transparency in a shader:
Here, the purple value field is purple because it has a driver which I've created by typing #frame into the field. This is a short cut for using the frame number. Set the frame value you want the object to appear as the second value field. If you're using this in EEVEE set the Blend Mode of the material to Alpha Clip.
You could type #frame == 25 directly into the Alpha input and it would create this driver:
but then it's pretty obscure what's going on.
Driver expressions are explained in the manual. You need to be aware of certain caveats but they are basically any Python expression that the driver can decode.