Anki has a main view, where your decks are shown. There is a menu bar at the top:
- Decks
- Add
- Browse
- Stats
- Sync
When you choose Browse (or type B, Browsing - Anki Manual) you can choose e.g. a deck from the sidebar. Now you will see a card/note count at the top of your window.
You can also choose Stats (or type T, Statistics - Anki Manual) to go the statistics screen. Here is also a total and per deck card count.