For any and every 11-cell pattern/shape, there are 39,916,800 possible combinations that an attacker could face just within that one pattern - e.g. A1 to A11, and 258,520,167,388,849,766,400 combinations if you use 23 cells. The checksum is calculated post hoc and pursuant to the 11-cells selected, and there are an additional 128 possible checksums for every 11-cell combination.

I haven't run all the numbers for the total number of possible 11-cell patterns but it is an extremely - extremely - large number. You would be far better off applying that energy to mining bitcoin directly.

Reply from Stack Wallet: https://twitter.com/stack_wallet/status/1687457709225099264?s=20

"We are implementing border wallets as is. However we've also come up with a novel approach for border wallets. But there seems to be a misunderstanding, it doesn't utilize a grid at all. We're writing up a paper that will be available for public scrutiny. We'll be sure to tag you. :)"

Hi - the ability to reproduce Entropy Grids using 12 words as entropy (128-bit/Deterministic Entropy Grids) uses Gibson Research Corporation’s Ultra-High Entropy Pseudo-Random Number Generator. Details here: https://www.grc.com/otg/uheprng.htm

This is a very cool project doing just this.