Correct spelling, explanation: approx. is an abbreviation for the adverb approximately. It’s a derivative of the adjective approximate. Since the base word is spelled with a double -p, the adverb approximately also features a double -p letter. It originates from the Late Latin approximatus.

Definition of approx.:
adverb, close to, around, in the region of
You listened to this song approx. ten times.
I would be there at approx. 7 pm.

Collocations with approx.:
Some most commonly used collocations include: approx. [umpteen] miles, and approx. [umpteen] percent.
I think we walked approx. ten miles.
Approx. 90 percent of these people don’t understand quantum physics.

Incorrect spelling, explanation: many English users tend to forget that the word approx. must be written with a double -p letter, because it’s an adverb based on an adjective. The adjective is approximate. Therefore, the form aprox. is a mistake and the only right one is approx.