How to remove commas from ALL the column in pandas at once

Numeric columns have no ,, so converting to strings is not necessary, only use DataFrame.replace with regex=True for substrings replacement:

df = df.replace(',','', regex=True)

Or:

df.replace(',','', regex=True, inplace=True)

And last convert strings columns to numeric, thank you @anki_91:

c = df.select_dtypes(object).columns
df[c] = df[c].apply(pd.to_numeric,errors="coerce")