No, that estimate is far too low (you'd need seeds of 1-2 orders of magnitude more apples)
The amygdalin contents of the apple seeds could gener-
ate between 0.06 and 0.2 mg cyanide equivalents per gram of apple
seeds; these values are relatively high. Acute cyanide toxicity can
occur in humans at doses between 0.5 and 3.5 mg kg⁻¹
weight (Speijers, 1993). In a previous study, Haque and Bradbury
(2002) reported the amygdalin contents of Fuji apple seeds to be
5.4 mg g⁻¹.
Bolarinwa et al.: Determination of amygdalin in apple seeds, fresh apples and processed apple juices, Food Chemistry, 170 (2015), 437–442. DOI 10.1016/j.foodchem.2014.08.083
1000 seed weight is 26.74 g according to Kew Gardens seed database.
0.06 - 0.2 mg CN⁻/g apple seed gives
1,6 - 5.3 mg CN⁻/1000 apple seeds
My old paper pharmacology and toxicology textbook (Estler: Lehrbuch der allgemeinen und systematischen Pharmakologie und Toxikologie, Schattauer 1990.) gives 1 mg CN⁻/kg body weight as lethal dose. The CDC info linked by the claim agrees with that.
Thus, for the standard human of 70 kg, about 70 mg of CN⁻ are lethal. I.e. eating (and chewing) 350 - 1170 g of apple seeds, which translates to roughly 13000 - 44000 seeds.
Now, I didn't find citable numbers of seeds per fruit for apples (the claim assumes 10 seeds/apple - which I judge is a plausible number), but for the claim that 20 apples contain a fatal amount of CN⁻ to be true, with the numbers above, each apple would need to contain 660 to 2200 seeds. I'd judge that this is about 2 orders of magnitude off.
Possible source of the error:
While the claim correctly cites the lethal dose of 1-2 mg/kg bodyweight their wording
1-2 mg/kg is a fatal oral dose of cyanide for a 154 lbs. (70 kg) man.
suggests that they apparently did not understand the meaning of dose given as amount per kilogram of body weight (1-2 mg/kg BW is independent of the body weight). According to the numbers given above, 200 apple seeds contain 0,32 - 1.06 mg cyanide. So this could explain the discrepancy (assuming the lower end of fatal dose range, and a comparably high estimate of amygdalin content in apple seeds).
- Another thing to keep in mind: amygdalin content is not equal to cyanide content. Amygdalin is a condensation product of cyanide, benzaldehyde and two glucose. 1 mmol = 457.4 mg of amygdalin yields 1 mmol = 26 mg of CN⁻. Taking amygdalin content for CN⁻ content would lead to estimates being a factor of 18 off.