The 20 on the 1d20 should be visible. The "D20" will be adjacent to the die. The font used for the D20 will be usable for our advertising purposes.
It shouldn't be too busy, something that can be etched or imprinted at a small size and still be recognizable is best.
These things are open to the artist interpretation:
The angle of the view
The other 19 faces can have the visible ones from the viewing angle be clearly showing their number, the other numbers are unclear, or the other numbers are missing.
The die should be recognizable as a die, but does not require all the qualities of a die. Nor is an issue if it does have all the qualities.
I included some googled (from the internet) samples showing dice and a couple "out of the box" examples. The examples and the original die are not forbidden, but the examples do show how not tackling the problem straight forward can lead to alternatives.