Johanna had 21 turtles - 5 turtles = <<21-5=16>>16 turtles.
After 1 month, Owen was left with 21 turtles * 2 = <<21*2=42>>42 turtles.
After 1 month, Johanna was left with 16 turtles / 2 = <<16/2=8>>8 turtles.
After Johanna's donation, Owen was left with 42 turtles + 8 turtles = <<42+8=50>>50 turtles.
#### 50