I guess you're talking about reference selectors.
Because if there is nothing selected in the first reference selector, the second reference selector isn't inititally constrained. So the second reference selector wants to show all objects, and that are in your case more dan 250 objects.
You can solve this by adding an XPath constraint. I will give solution for the following case:
- A dataview with object A
- A reference selector (the first selector) in that dataview for selecting B (relation a-b, 1-0)
- A reference selector (the second selector) in that dataview for selecting C (relation a-c, 1-0)
The last selector is constraint by the first because of the relation from C to B (c-b, 1-0)
In this case you can solve this issue by adding an XPath constraint to the second reference selector as follows: