It looks like you have element instances, but want to set values on the element type. Just like Revit requires you go into the Type Properties dialog, Dynamo is going to ask you to do the same. Using an Element.Parameters node will likely illustrate that to some extent.
Try looking for ways to get the type as a start (it’s pretty easy to find), and note that type parameters can only have one value so you may need to use a List.Unique as well. Once you have unique types, feed the list along to count them, add the relevant prefix, and finally set the parameter value for the the type elements not the instance elements.