A system of 3 linear equations with 3 variables has an augmented matrix whose row echelon form is $\begin{pmatrix} 1 & 2 & 1 & | & 0 \\ 0 & 1 & 3 & | & 0 \\ 0 & 0 & 0 & | & 1 \end{pmatrix}$. How many solutions does this system have?