Add Rows/Columns to a Grid dynamically
To have a dynamic Grid size(column/row count), you can use helpers such as @Rachel 's WPF Grid’s Row/Column Count Properties
In your case, this depends on what exactly your trying to achieve as in most simple cases based on context, when there is a requirement such as:
Number of child user controls that need to be populated in the parent user control are determined at run time and can change. Basically the number depends on the search result. So it can be from 0 to n.
I'd prefer to go with an ItemsControl (Example). Use the ItemTemplate to link to your child UserControl's. If needed you can also tweak the ItemsPanel to control layout as you see fit.