How many unpaired electrons are there in a strong field iron(II) octahedral complex?

There are zero unpaired electrons in strong field Fe(II) octahedral complex.

Electronic configuration of $Fe$Fe is $\space3d^6\space4s^2$ 3d⁶ 4s²

Electronic configuration of $Fe(\Iota\Iota)$Fe(II) is $3d^6\space 4s^0$3d⁶ 4s⁰

Because, 2 electrons are removed from 4s orbital of the Fe(II).

In a strong field, d⁶ electrons are split into $t_{2g}^6$t₂g^6​ and eg⁰​ . These 6 electrons in t₂g​ are paired. So there is no unpaired electron in the octahedral complex.