Most splitters make you think in rows and columns. This one does not. You say how many pieces you want and it picks the arrangement.
The rule is simple: it finds the factor pair closest to square and uses that. 4 becomes 2 by 2. 6 becomes 2 by 3. 12 becomes 3 by 4. 16 becomes 4 by 4. A prime number has no factor pair, so 7 becomes one row of 7 and 11 becomes one row of 11. If the arrangement it picks is not the one you wanted, every alternative is one click away, and the count stays the same.
Equal means equal to the pixel wherever the maths allows it. A 1080 × 1080 px image into 4 gives four 540 × 540 px parts, identical to the pixel. When the numbers do not divide cleanly, the cuts land on whole pixels and the last part in each row and column absorbs the remainder, which is at most one or two pixels. That is the difference between parts that rejoin exactly and parts that leave a hairline seam.
The ceiling is 20 by 20, so 400 pieces. Realistically anything past about 30 pieces is a job for splitting by tile size rather than by count.
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