data_matrix = id_df[seq_cols] ; num_elements = data_matrix.shape[0]

Explanation:

The first line of code assigns a subset of columns (specified by seq_cols) from the DataFrame id_df to the variable data_matrix.

The second line of code calculates the number of rows in the data_matrix using the shape attribute, specifically the first element of the tuple representing the shape. This is assigned to the variable num_elements.

Overall, these two lines of code are preparing the data for the next steps in the analysis, which likely involve some form of modeling or statistical analysis.