You are asked to dissect an $N \times N$ square into polyomino pieces such that each piece shares a portion of its boundary with exactly $D$ other pieces, and no piece has area exceeding $N$. This can be achieved for $D \le 5$.
For $D=2, 3, 4$ the smallest such squares are of size $2 \times 2$, $3 \times 3$, $4 \times 4$, respectively:
Find the smallest square for $D=5$.
Credit: inspired by this puzzle.
EDIT / Bounty offer by Will Octagon Gibson:
Two answers have already been posted using 8x8 squares and 16 polyominoes.
I plan to award a bounty of 100 if a correct answer is posted using a smaller square or fewer polyominoes.
I plan to increase the bounty to 200 if a correct answer is posted using a smaller square AND fewer polyominoes.
These offers will expire on November 30, 2023 at approximately 11:59 p.m. (my time).
Good luck!