After finishing season 4 this question got stuck in my head so I watched the Lab scenes again and count the children in the Lab many times.
There are 16 children in total. Number 1 is an adult now. And number 8 isn’t in the Lab. (in episode 6 number 1 says when 8 was still here, that means she isn’t in the Lab anymore).
16 children + Number 1 + Number 8 = 18
This means the last childs number is 18. But why is this important? During the DnD game when Dustin rolls the dice he gets 11 but that’s not enough to beat Vecna. When Erica rolls the dice she gets 20 and that’s enough to beat Vecna. A lot of fans (including me) thinks this is foreshadowing. Sooooo Eleven isn’t gonna beat Vecna by herself she is gonna get help from another child with powers.
There is also another theory about a child whose number is 20 gonna beat Vecna but there aren’t 20 children in the Lab. I think some children survived and we are gonna see them on part 2.
