Requirements for a clan war?

The only requirement is that a clan must have at least 5 members eligible for war. (This used to be 10, but 5 vs 5 war size is also available now.)

Members who are new to your clan will be ineligible for war if they left their previous clan while a war was in progress. They will become eligible for war again when the previous war is completed. You can check your clan member list to see if any members are ineligible for war, and see when they will be eligible again. If you start the clan war while members are still ineligible, those members will not be able to participate.

You can see when players will be eligible in their profile through clan's chat room.

Clan War eligibility is simple:

• Your clan must have at least 10 members.
• You can only wage wars in increments of 5, giving us the following available wars:
• 05 v 05
• 10 v 10
• 15 v 15
• 20 v 20
• 25 v 25
• 30 v 30
• 40 v 40
• 50 v 50
• If your clan has more members than is required for war, then the eligible members with the highest trophy count are chosen.
• This is likely what's causing your confusion. If you have 12 members, the 2 members with the lowest trophy count won't be able to participate.
• Members will be ineligible if they left a different clan after a war was started (for which they are participating in), they will be eligible again once that war ends. You can view their profile to see when they will be eligible.