How is the amount of zakat on sheep for the past eight years calculated, given the husband's ignorance of its obligation and his lack of precise knowledge of their number each year?

Your husband must pay zakat (definition) on the sheep he owns for trade, if a hawl (lunar year) has passed and they have reached the nisab (definition) (595 grams of silver). Zakat is to be paid at a rate of 2.5% of their value. The hawl begins from the date he acquired the money with which he purchased the sheep, if that money reached the nisab; otherwise, it begins when the value of the sheep themselves reaches the nisab. Your husband erred by not paying zakat in previous years. He must repent and pay zakat for all past years, by valuing the sheep at the end of each hawl and paying the due zakat. You may pay the zakat on his behalf, provided you inform him and he agrees, because intention is a condition for the validity of zakat.