The Missing 28

 43 super children were all born at precisely 9:38 P.M. on an unknown date in 1950. 

Eight of these children were adopted by Sir Reginald Hargreeves.

1 Luther Hargreeves: Space Ape

2 Diego Hargreeves: The Horror

3 Allison Hargreeves: The Rumor

4 Klaus Hargreeves: The Seance

5 Five Hargreeves: Portal Boy

6 Ben Hargreeves: The Kraken

7 Vanya Hargreeves: The White Violin

8 Viktor Hargreeves: The Musician

In the new timeline, an alternative seven were adopted instead. One of whom, Ben, was an alternate version of the original Ben.

1. Marcus Hargreeves: The Muscle

2. Fei Hargreeves: The Unkindness

3. Alphonso Hargreeves: The Shield

4. Sloan Hargreeves: The Psionic

5. Jayme Hargreeves: The Snake

6. Christopher Hargreeves: The Cube

Plus Lila who was brought up by The Handler. 

1. Lila. 

8 + 6+ 1 = 15 

43 - 15 = 28.

These are The Missing 28.