You can do that by handling the LinkCreating and LinkModifying validation events. E.g. this disables drawing an outgoing link from a node when the node already has at least one link:

private void diagram_LinkCreating(object sender, MindFusion.Diagramming.LinkValidationEventArgs e)
{
     if (e.Origin.OutgoingLinks.Count > 0)
           e.CancelDrag();// or set e.Cancel = true;
}

The difference between calling e.CancelDrag() and setting e.Cancel = true is that the former immediately stops the drawing operation, whereas the latter changes the mouse cursor to a stop icon but allows the user to continue drawing the link to a different location.

I hope that helps,
Stoyan