Anki doesn’t show all of the cards in the deck in study mode

I have 104 cards in my deck, but when I look at the overview of all decks it shows me that there are only 64 cards in the deck. At first it was 20, then I changed the settings concerning the maximum of new cards shown per day to 500 and now it’s at 64.

I don’t understand how I can change this so I can view all of my cards.

Your are talking about different things. What you are referring to is probably the main deck view, where you see all decks with their scheduled review numbers. These numbers only show, as indicated, the number of cards scheduled today. There are several factors that could prevent a card from being scheduled today. On being the maximum number of new cards shown per day, but not only. If a card is suspended (maybe because it was a leech, or maybe because it was manually suspended) or if it is buried (because it is a sibling card of an already scheduled card and that the deck option Bury Siblings is checked, or because it was manually buried), it won’t show up.

This behavior is normal, and you usually are not expected to review your whole collection every day: Anki’s purpose is to be able to memorize way more content than you can go through in a single day.

That being said, if you just want to see your whole collection, not review it, you can access it through the browser.

For some more info, please see Anki's not showing me all my cards! - Frequently Asked Questions