You're talking to a former Spidey completist here crops. 6 years ago I had everything and had 8,000 comics. That included pence, Canadian, Australian, MJI's, variants, subsequent printings etc. Every title. Every mini series. Every variant. Every one shot. Every appearance. Continuity, non continuity. USM, SM 2099 etc etc etc. And still only 8,000.
The 12k figure is meaningless without parameters. ASM #666 has 145 variants. Do they count as one?
How are they defining appearance? I included Spideys face on a kids t-shirt in the background. Cover appearance only or story? Parker only or Morales?
I'm leaning towards dismissing the list as meaningless.