Based on my understanding, you will only need to have staked SPS to buy Chaos Legion packs if you are buying in the pre-sale.

So to buy 1 pack during the pre-sale, it will depend on how many voucher tokens you have earned which is based on the percentage of the SPS you have staked out of the total global staked SPS. 1 Voucher token = 1 Pack.

The question therefore should be, how much staked SPS do I need to earn 1 or more voucher tokens a day?