B

Step 1: Identify the coordination number and geometry. The given complex is [Pd(NH3)2(H2O)2Br2]^{2+}. Here, palladium (Pd) has a coordination number of 6 (2 ammonia, 2 water, and 2 bromide ligands).

Step 2: Determine the type of isomerism. With a coordination number of 6, the complex can exhibit geometric isomerism (facial and meridional) and optical isomerism if applicable. This is because the arrangement of ligands around the central metal can create different spatial orientations.

Step 3: Calculate the maximum number of stereoisomers. For octahedral complexes with two types of bidentate ligands and two identical monodentate ligands, the general formula for the maximum number of stereoisomers is 2^n, where n is the number of ligands that can create different orientations. Here, n = 2 (bidentate ligands) and 2 (monodentate ligands), leading to a total of 3 stereoisomers (fac and mer) plus their optical isomers. Hence, we arrive at a maximum of 6 spatial isomers.

Therefore, the maximum number of stereoisomers possible for the ion is 6. Thus, the correct option is B.