To count the NaN values in a column in Python Pandas DataFrame, we call isna and sum.
For instance, we write
s = pd.Series([1,2,3, np.nan, np.nan])
count = s.isna().sum()
to create a series witth pd.Series.
The we call isna to return the isna values in the series.
And then we call sum to get the count of them.