Considering H2O as a weak field ligand, the number of unpaired electrons in [Mn(H2O)6]2+ will be (At. No. of Mn = 25)

Mn25→3d5+4s2Mn2+→3d5In presence of weak ligand field, there will be no pairing of electrons. So it will form a high spin complex. i.e. the number of unpaired electrons = 5.