I will discuss how many exact Lagrangian fillings of Legendrian torus knots are possible up to Hamiltonian isotopies. The main tools are augmentations of Legendrian DGA and Lagrangian saddle cobordisms. I will also talk about how the cluster structure and Dynkin diagrams are related to this subject. This is a joint work in progress with Byunghee An. Lecture 3: Cluster structures and space of augmentations.