〜に〜回
… times per …
Period + に + number + 回
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To say how often, put に after the period and the count after it: 一日に三回, "three times a day"; 一週間に二回, "twice a week".
回 (かい) counts occurrences: 一回 (いっかい), 二回, 三回… and 何回 asks "how many times". 度 (ど) is a slightly more formal alternative: 一年に一度, "once a year".
The same に-for-a-period appears with other counts: 一時間に電車が六本あります, "there are six trains an hour".
Don't swap in で: 一日で三回 would mean "three times within a (single) day", about finishing something, not a regular rate.
Examples
一日に三回、薬を飲みます。
I take the medicine three times a day.
一週間に二回、ジムに行きます。
I go to the gym twice a week.
一か月に一回、映画を見ます。
I see a film once a month.
一年に一度、国へ帰ります。
I go back to my home country once a year.
一時間に何本電車がありますか。
How many trains are there an hour?