what would be the Maximum number of nodes goxam support in Silverlight
That depends on how complicated each node is.
Does your Node DataTemplate have a lot of elements in it?
Does it make use of Controls? (Lists and grids are inherently expensive in space and time.)
Does it have a lot of data-binding in it?
It seems unlikely that your application is permanently unresponsive. Is it just slow to load? That might be because the nodes are complicated. Or that might be because of the diagram layout that you are using. LayeredDigraphLayout currently does not scale well.
If you data-bind the go:Node.Location to a node data property, and if every node data of yours has a location initially (i.e. you don’t need a Diagram.Layout), you could try virtualizing the construction of all of the nodes and links.
I’ve been intending to write a forum post about this, so I’ll do that next.