Don’t introduce them all to us at the same time, even if they’re in the same scene. And take the time to figure out which characters are most important, i.e. “secondary characters” – and which are mostly there for background – i.e., “tertiary characters.”
Secondary characters would be those like Hermione Granger, Ron Weasley, Professor McGonogall, and Albus Dumbledore. These characters require more introduction, so give theme a scene – even if it’s a small one – where they interact with a main character. Limit yourself to having two secondary characters in an introductory scene (like the train scene in Harry Potter where we meet both Ron and Hermione).
Tertiary characters don’t need as much introduction, but the reader also doesn’t need to know as much about them. Example – Neville Longbottom. But if that character becomes more important later on in the story, like Neville does, then you can have a more proper “introduction” to them (such as when Harry learns about Neville’s parents).