Specialized CSV processed python

I think this article can be applied.
Add to CSV in Pandas

An interesting feature for storing data is the append mode with the parameter a, which you can use to add data to an existing CSV file.

pandas.DataFrame.to_csv

mode:str
Python write mode, default 'w'.

You can prepare two DataFrames like this, write each DataFrame separately, and put the second in append mode.

import pandas as pd

df1 = pd.DataFrame ([1]], columns = ['Number of Lines'])
df2 = pd.DataFrame ([10, 20, 30]], columns = ['Column A', 'Column B', 'Column C')

outfilename = 'combined.csv'

df1.to_csv(outfilename, index=False, encoding='utf8')
df2.to_csv(outfilename, mode='a', index=False, encoding='utf8')

Well, the first two lines could be just a two-line text file, not DataFrame.