。
I brush my teeth twice a day.
Literal
I [topic-は] 1-day [per-に] 2 times teeth [object-を] brush.
The frequency pattern [time unit]+に+[count] means 'per [time unit]' — １日に２回 = 'twice per day.' This is the standard way to express rates in Japanese. 歯を磨く ('to brush teeth,' literally 'polish the teeth') is the fixed expression.