The number of Clan Cards you can get on the first day based on your Clan War League and your match results. You use your own cards to play on the Collection day. Once a game mode played, it will be replaced by a different game mode. There are different game modes here for you to choose: 2v2 Ladder, Sudden Death, 2v2 Double Elixir, Draft Mode and Double Elixir.
During the first day, you have 3 matches to play in the War Map (screenshot above). The first day of the Clash Royale Clan War is the Collection Day. Test it either with your friends, Challenges or even Ladder matches. Always test and play the War Deck before the War Day.You will know exact the cards you should upgrade.