Использование np.where -
df['market_share_2'] = np.where(df['Item Type'].values=='Baby Food', 'Baby Food', 'Others')
Выход
Item Type market_share market_share_2
0 Office Supplies 10 Others
1 Baby Food 20 Baby Food
2 Vegetables 10 Others
3 Meat 30 Others
4 Personal_Care 10 Others
5 Household 20 Others
Затем используйте value_counts()
-
df['market_share_2'].value_counts()
Others 5
Baby Food 1
Name: market_share_2, dtype: int64
TLDR;
pd.Series(np.where(df['Item Type'].values=='Baby Food', 'Baby Food', 'Others')).value_counts()