When making this card, my idea for this is that it has NO UPPER NUMBER CAP, but there is a limit of how many Mists you can have. Then, at most, with 10% block per mist and 23 Mists on the field, you'd have 92% block rate, which is almost invincible; but Mists are very easy to get rid of.
I don't want this to be a mist-counter thing like Bone Wall. I think that you should actually pump Air quanta into this thing to make more Mists.
Making the Mists into Permanents is an option... But then, there is no easy and fast way to get rid of an entire field of Mists. If you have 14 Mists (the 15th slot is for Pillar, of course), then your opponent has 80% miss chance and it'd take them a long time to destroy them all. Making the Mists stack would be even worse, because that way there will be NO upper limit to the number of Mists you can have.