。
Mother divided the cake into eight pieces.
Literal
Mother [topic-は] cake [obj-を] eight [into-に] divided (polite).
The pattern X を Y に分ける ('to divide X into Y parts') is straightforward, but the counter ~つ deserves attention here: ８つ reads やっつ, the one place in the ~つ counter sequence that breaks the otherwise predictable pattern. Specific numeric choices in Japanese sometimes carry cultural weight too — eight (八) is broadly auspicious because the kanji's spreading shape suggests prosperity widening — though here it's probably just logistics.