Why does the fact family with the number 8 and 16 only have 2 number sentences instead of 4?

Perhaps because 8 is half of 16, the addition and subtraction facts can only be written one way each.
  16 - 8 = 8
  8 + 8 = 16

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There are more number facts involving 8 and 16. It is helpful to know which of them you are concerned with.
  16/8 = 2
  16/2 = 8
  8*2 = 16
  2*8 = 16
  16*8 = 128
  128/16 = 8
  128/8 = 16