I don't think the BCX will be 64m in 5 months.
Why? Staking SPS will be required to earn rewards as rapidly as we do now. This means that unless there is a material increase in the amount of SPS staked, fewer reward cards will be issued due to there being fewer RP / SP distributed by those who earn in a league or level where they do not reach the staking requirements.
I like the idea another comment had : if the card is soulbound, it requires more DEC to be staked.