Vertical box - children limit

Hey, I have a vertical box which all of its children are auto size inside the box.
If you will add too much children eventually they will be out of the vertical box size:
TYrL2eCMRjqHCDl2kJ7p6Q.png

How can I check if there any more room to add more children?
By the way I have tried the function “CanAddMoreChildren” but it does not work.