、。
Since there are a lot of people, we decided to make today's event a standing buffet.
Literal
People-number [subject-が] many is-[polite-です][because-から], today [topic-は] standing-meal format [as-に] did-[explanatory-んです].
Explaining a hosting decision. 人数 ('number of people') takes the 多い/少ない adjectives for quantity (vs. 多くの人 for 'many people' as an attribute). 立食形式 ('standing buffet format') is a set compound for a standing-reception-style meal, contrasted with 着席 ('seated'). ~にする ('to decide on X, go with X') marks a choice made by the speaker. ~んです adds explanatory nuance, showing the speaker is providing the reason behind their decision.