The equivalence relation $(a, b) R (c, d)$ is defined by $ad = bc$. For $(3, 2)$, the equivalence class consists of all $(c, d)$ such that: \[ 3d = 2c. \] Since $c, d \in \{2, 3, 4, \ldots, 18\}$, solving for integer pairs $(c, d)$ satisfying $3d = 2c$ gives six solutions.