There's a couple of tabs which lock into slots on the outer edge.
Provided they're home, there's little point in adding a screw on that side.
One screw is sufficient to hold the grille in place, you won't lose it!

If you're concerned about corrosion, use a stainless screw (or cable ties).

Only a single small one on my later one-part version and pretty insubstantial as well, really just to position it correctly against the bumper outboard of the foglight.
There's nowhere in that area suitable for a screw anyway, so I'll leave it alone.

I did mine today with black screw in each top corner, following HP's wise choice of location

Drivers one looks rock solid but passenger one had a bit of movement

Bit of piece of mind now +++