So you're writing a book with many, MANY characters. How do you make sure you keep track of all of them? How do you keep yourself from boring the reader with lengthy descriptions of each person's physical attributes? How do you remember when and where you mention a character?
Well the first thing you need to do is MAKE NOTES! I'll give you an example from my current WiP: I created a sports league with twelve teams made up of eight players per team. That's 94 characters I need to create (because the two protagonists are also players in the league). I'm handling this by writing down the complete league roster.
Any time I mention these members, I'll make a note of which chapter so that I can find the context without difficulty. To be fair, most of the time a character is mentioned, it will be in the context of their team, but if you're writing a longer book, make more detailed notes.
But now the big one: physical attributes. Question: what does Dean Thomas look like? And don't describe the actor who played Dean Thomas in the Harry Potter movies. What about Katie Bell? The Patil twins?
Some of these characters played a significant role in the Harry Potter series, but we know very little about them based on the descriptions given. THAT is how you keep the story from getting bogged down. The protagonists – the characters always on the page – they're the ones who need to be properly three-dimensional. The other people are set-pieces. They need to come across as real people when they interact with the main characters (and when you make decisions on personality traits, note those in your character list), but otherwise, don't spend a lot of time on those cardboard cutouts.
Just make sure you show them from the proper angle.