It's not really an answerable question.
There's so much to know, and as others have pointed out very few people even come close to knowing a lot about every set. There's different sorts of knowledge too.
If it's about what cards are worth- just pick any 10 guys who have been full time dealers for 25-30 years or so and you'll be close enough.
But many of those guys may not know some of the finicky details that someone who specialized in one set or series might know. Not because it's not worth knowing, but because it hasn't been important to them to know it.
Some knowledge gets just odd. I recall One of the Halls saying he could identify a topps card -year and player- while only seeing about 1/8th of the card, sometimes less. He actually did it, at least for the 20 or so cards I tried. All from 59-79, and I only showed about one square inch of card!
Yeah, I spent way too much time hanging out at the shop, but I learned a lot in the process
Todays access to information is amazing. In a short time we can get info directly from someone with a great deal of knowledge, and usually with a picture too.
I'm always amazed at the volume of knowledge the hobby pioneers gathered with not much more than paper pen and a few stamps.
Steve B