Assume $\mathrm{H}{}_2\mathrm{O}$ is a weak-field ligand here. The number of unpaired electrons in $[\mathrm{Fe}(\mathrm{H}{}_2\mathrm{O}){}_6]{}^{2+}$ is

Fe is $+2$, so the ion is $d^6$. In the weak field (high spin) configuration $t_{2g}^4{e}_g^2$, the first five electrons are unpaired and the sixth pairs up, leaving $4$ unpaired electrons.