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Can you play action cards in your opponents turn when you’re responding to their attack and secondly can you have as many battle cards in your deck as you want, as long as it’s a minimum of 40?

No, you may only play Actions if you have an Action play which is generally during your turn before you attack. You can have as many battle cards as you want over 40 as long as you can reasonably manage them.
Source: Wizards of the Coast (2019-02-11)    【Link to Ruling】
Tags: Play, Deck