Considering ${\mathrm{H}}_2\mathrm{O}$ as a weak field ligand, the number of unpaired electrons in $[\mathrm{Mn}{{\left({\mathrm{H}}_2\mathrm{O}\right)}_6]}^{2+}$ is

$\mathrm{Mn}3{\mathrm{d}}^{5}4{\mathrm{s}}^2$

${\mathrm{Mn}}^{2+}3{\mathrm{d}}^5$ (i.e.) 5 unpaired electrons because ${\mathrm{H}}_2\mathrm{O}$ is week field ligand and does not force to pairing.