You can count the number of members in a list. This is useful for determining how many times the end-user repeats over a loop, or for determining the number of data sets that's retrieved from a Workbase with a Query Block.
For example, here is how you it works for a list variable craeated by a loop:
The variable created by your Expression Block (in the example above, the variable is number_of_children) is equal to the number of list members.