len(x[0])
Since you want to get the 1st dimension length
In other words,
len(x) gives number of items at 0th dimension
For e.g.
> x=1,2
> len(x)
2
> x=1,2,3
> len(x)
3
To get the number of rows in the first item
len(x[0])
To get the number of columns in the first item and first row
len(x[0][0])
To get the number of columns in the first item and second row
len(x[0][1])
So on and so forth
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