The Clan War rewards system will take into consideration the amount of kills made by the clan and the total number of trophies during the current war. The number of kills will define if the clan wins or loses the war and will also set how many chests the clan will receive upon winning.

The Clan War reward chests can level up, and higher levels will give you more prizes! To increase your rewards you must first reach a new level, which can only be unlocked after the clan achieves a specific amount of trophies. When the clan gets the necessary number of trophies to unlock a reward level, it will unlock a higher reward for the next Clan War. For the following Clan War, the clan must then make a specific number of kills to receive the new rewards.

So you need both trophies and kills to unlock new rewards. If the clan has 100 trophies and makes 1000 kills, you will receive level 2 rewards. If the clan has 100 trophies and makes 3000 kills, you will still receive a level 2 reward. To receive level 3 rewards the clan must have at least 300 trophies.

For example:
If your clan had 750 trophies when you joined the Clan War, and you have made 30,000 kills in the current war, you should receive the level 6 chests because you have met all the requirements.

Another example could be that your clan has 740 trophies at the moment you join the Clan War. The level 6 chests will not be unlocked unless you achieve 750 trophies. If your clan wins the war, you will receive trophies for the victory and it will unlock the level 6 rewards. In this case, even if the clan has made 30,000 kills in the current war, you will still receive level 5 chests. Then, when your clan joins the following Clan War, the rewards will be already unlocked, so you just need to make the minimum number of kills to receive it!