It depends on what kind of element you are using. For example in solid2D plain strain / plain stress with 2 dofs per node (ux, uy), the node number I is located in

[(2I-1, 2I-1) (2I-1, 2I)

(2I, 2I-1) (2I, 2I) ]

You can locate nodes manually or automatic. In manual discretezation you can divide the domain into subdomains and each subdomain is discrtisiezed into elements to an acceptable degree of accuracy then you locate nodes at each element heads. The same way you can do automatically by defining the number if devisions in each boundary. In this way mesh elements are developed with nodes at the cornors of each element

Abdelghani Seghir your solution is applicable when the mass or stiffness matrix is extracted by *DMAT command. While in this problem I could not get dense matrix regarding large number of nodes and elements. I used *SMAT command to extract a sparse matrix and I need to locate nodes in this sparse matrix. The formula you mentioned is not correct for a sparse mass matrix

Could you transfer your matrices to MATLAB. If yes it will works