How many different possible structures (isomers) are there for the complex, [Mn(OH2)3(NH3)3] 3+?
Metal complexes with three each of two types of ligands $(\ce{MX3Y3})$ have two isomers: mer- and fac-isomers. For example, for $\ce{Co(NH3)3Cl3}$ there are two isomers:


  Fac for facial, in which all three ligands of each type are clustered on a single face of the octahedron and the three other three ligands are on the opposite face.





  Mer for meridional, in which the three ligands of each type are arranged meridianally in a row around the metal and the other three ligands are arranged likewise on the opposite side in a perpendicular meridian.