Bears overturn rocks to find the insects, as many as 40,000 per bear in a single day.
Here, the modifier is green is nonessential and ends the sentence.
For this reason, only one comma is required, as per the discussion above.

OA: The army cutworm moth is a critical source of fat for as many as a third of Yellowstone National Park's grizzly bears, which overturn rocks to find the insects, as many as 40,000 per bear in a single day.
Here, the second comma is required for two reasons:
Because the sentence extends beyond the nonessential which-modifier in red, a second comma is required to enclose the nonessential which-modifier.
The as-modifier in green is nonessential and thus must be preceded by a comma.
Thus, the second comma serves dual purposes.

Sir,
Thanks a lot for clearing my doubts.
I need further help from your in the following doubts.

In the following sentence, "as many as 40,000" is referring what? ......... rocks or insects? what's the way to find the the actual reference here.
"Bears overturn rocks to find the insects, as many as 40,000 per bear in a single day"

If "as many as 40,000" is referring to insects, what's the main reason to use comma here?
Would the following sentence be wrong?
"Bears overturn rocks to find the insects [without COMMA] as many as 40,000 per bear in a single day".

Please some examples in which rocks can be referent for such construction.

Regards
Belal