Chirality and number of sterioisomers
I believe you are misreading the question. Rather than asking for the number of stereoisomers the product can have (this is where 2^n comes in), it is asking for the number of stereoisomeric products formed. Note that that the two products are diastereomers, which is a type of stereoisomeric relationship. It's not looking for any fancy calculation; just for the related products.

In a Gringard mechanims, the Gringard reagent can attack the carbonyl on either the re or si face, which in turn, can yield an R chiral center or an S center at the location of attack. This lends to the diastereotpic products.