For a structure with \( n \) degrees of indeterminacy, the number of plastic hinges formed at complete collapse is equal to the degrees of indeterminacy plus 1. This is based on the general plastic collapse theory for structures.
Given that the structure has 2 degrees of indeterminacy: \[ \text{Number of plastic hinges} = n + 1 = 2 + 1 = 3 \] Thus, the number of plastic hinges is 3.