Each image has 64 pixels. Each pixel is a feature, so we have total 64 features for each image that should be put in one row of the feature matrix.

So, the total number of rows in your feature matrix is equal to the number of images (each row for an image), and the total number of columns in your feature matrix is equal to the number of pixels (64).

If you have a list of arrays with 64 elements (64 pixels), then you only have to concatenate them in order as rows of a pandas dataframe. You can also make a dataframe and add rows one by one.