Mostly so other can guess first, without accidentally seeing the answer.
I know 11. I’m 44.
About three or four of the others I’m familiar with their work but didn’t know their name.
I don’t know Bruce Forsythe.
I recognize the names of Charles Aznavour and Juliet Prowse but I couldn’t tell you a specific thing they were in.
I’m sixty.
All of them except Bruce Forsyth. I’m 59.
I think I recognize six, but it might be fewer, if I misidentified some of these:
Jim Nabors: Singer, but best known as Gomer Pyle
Lena Horne: Wasn’t she the one in the commercial that started “I’m Lena Horne. Guests are visiting from Europe…”, and everyone in the US went “Who?”?
Candice Bergen: She was definitely on a sitcom, but I can’t remember which one.
Phyllis Diller: I think she was a comedian?
Twiggy: Nickname for a very thin fashion model.
Ethel Merman: Wasn’t she the neighbor on I Love Lucy?
I’m 45.
Bruce Forsythe and Charles Aznavour are 2 of the only 4 i know !!
But then i’m from the UK…
(And 60 yo.)
Juliet Prowse, I know the name, but not sure who she was. Dancer or singer?
Connie Stevens: Yes
Joel Grey: Yes
Ruth Buzzi: Yes
Rita Moreno: Yes
Jim Nabors: Yes
Florence Henderson: Yes
Paul Williams: Yes
Charles Aznavour: No
Harvey Korman: Yes
Lena Horne: Yes
Peter Ustinov: Yes
Bruce Forsythe: No
Sandy Duncan: Yes
Candice Bergen: Yes
Avery Schreiber: Yes
Ben Vereen: Yes
Phyllis Diller: Yes
Vincent Price: Yes
Valerie Harper: Yes
Twiggy: Yes
Ethel Merman: Yes
Kaye Ballard: Yes
Mummenschanz: Yes
Mid 50s
I’m old and I recognize this as a list of people who guested on the Muppet show (and I recognize 20 names off the top of my head - I’d probably recognize the faces of the others) (mid-50s, by the way)
That makes sense, my daughter didn’t recognize many, but those she did were mostly from the Muppets or Scooby Doo. (I brought her up right).
All but 2. I’m 43. I looked at the spoilers and it makes sense, but I only really know Paul Williams because of Yo Gabba Gabba. I have 2 teenagers. I’m probably around the age that would recognize most of the names without making the connection and then have exposure due to having kids in the past decade, a slightly older spouse, and nostalgic pop culture.
I know the commercial you’re talking about (some “international coffee” line of instant coffees?) but pretty sure it wasn’t Lena Horne, it was some foreign actress. Rula Lenska?
Jim Nabors
Florence Henderson
Sandy Duncan
Candice Bergen
Twiggy
Phyllis Diller
Vincent Price
Ethel Merman
John Denver is the only person I can remember from The Muppet Show
Early 50s
8 for me ( familiar with the work of 8, though the names of about 8 more sorta’ ring a bell )
Age 60
Ditto. I’m 59. I didn’t pick up on their connection, though.
Dancer and actress, though by the '70s, she was primarily known as a dancer, as well as for having shapely legs. At that point, she was doing TV ads for L’eggs pantyhose, and Mannington flooring. I also remember her as the host of televised ballroom dance competitions, which used to air on PBS.
Also, as a kid who grew up in the '80s several of these headlined primetime shows (Valerie Harper and Sandy Duncan were the lead on the SAME show at different times, and Duncan was on some nonstop cracker commercial) then. Everyone knew Murphy Brown. And/or were stars of popular reruns (Harper again, Jim Nabors, Henderson, and more), had 2nd (3rd?4th?) acts on kids shows like Ben Vereen. Were stars of timeless classic shows or movies like Rita Moreno. Or were just considered “legendary” or entered the “famous for being famous” commercial and guest tv spot phases of their career.
21 of them, and I do know who Charles Aznavour was. 
I know the names because of things other than the purported origin of the list, as I seldom saw that show.
I’m 64
8 I’m 52.
I only know Lena Horne bc I saw her on Sanford and Son.
I’m 52 and recognize a little over half of them. I also remember them being people that adults at the time thought were funny/talented but that I as a kid in the 70s thought were completely unfunny.
Me as an 8 year-old watching the Muppet Show: “Who is this schmuck they have guest hosting? Somebody go get Mark Hamill.”
Murphy Brown. Also known for being the daughter of famous ventriloquist Edgar Bergen (who was also a guest on The Muppet Show), for being an early guest host of Saturday Night Live, and for being a regular on Boston Legal.
Not to be confused with Ethel Mertz, Ethel Merman was a Broadway singer/actress known for her “belting it out” style of singing. Had a memorable cameo in Airplane!