The formula $Al_2SO_3$ represents aluminum sulfite. To find the number of atoms, we need to break down the formula:

* Al: Aluminum has a subscript of 2, meaning there are 2 aluminum atoms.

* S: Sulfur has a subscript of 1 (implied), meaning there is 1 sulfur atom.

* O: Oxygen has a subscript of 3, meaning there are 3 oxygen atoms.

Therefore, there are a total of 2 + 1 + 3 = 6 atoms in a molecule of aluminum sulfite ($Al_2SO_3$).