一度に + [verb]
一度に means "all at once", "in one go", "at a time": a quantity is dealt with in a single go rather than in parts. It is an adverb and goes before the verb, often with a word for how much: ぜんぶ全部, たくさん, a number.
Without に, いちど一度 counts: "once", "one time". 一度に is about doing a lot in that one time.
With a number and a potential verb, 一度に gives a limit: how much fits or can be handled at one time.
Its opposites are すこしずつ少しずつ (little by little) and ひとつ一つずつ (one by one).