In the case that there are 250 people, we will partially use a regular service.
If there are 250 people we will partially use a regular service.
「の場合」- in the case that; in case; if
「一部利用」- Part use； partially use(動詞)
「定期便」- Regular service (flight, bus, train etc)
「In the case that there are 250 people, we will partially use a regular service.」250名の場合、定期便を一部利用
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