10,000,000 (10^7).
The array used in an mmult formula is typically a square array. The impact of the 10 million cell limit on a square 2D array is that you can have sqrt(10^7) rows and the same number of columns. In other words, you can process 3162 rows of data using an mmult technique that produces a square 2D array, but not 3163.
With
=sum(sequence(3162,3162))
With
=sum(sequence(3163,3163))
As noted by @player0, many other combinations of height and width are possible in rectangular arrays from 10m rows X 1 column up to 1 row X 10m columns as long as the total number of cells does not exceed 10m.
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