I think I had two cards as well by the time I bought my last one. I then reshuffled, having not enough iMG left and wanting to get a cheaper card to buy. No further brain cards appeared. It took me quite a while to realise that this was because I had hit the cap (that I hadn't known of beforehand).
I orchestrated things so that I had 3 +1 brain capacity cards in my hand at 59 (in the hope of getting to 62).
Unfortunately, while it appeared that I was able to buy all 3 cards, I didn't actually get +1 brain capacity from the latter two but some other random cards. That is, it looked like I was able to purchase the extra +1 brain cards but when I did, I actually was charged for and received different cards (an energy tank upgrade and possibly a quoin upgrade).