total number of modules answer is 25. Now i will provide 2nd answer what is best for freshers. It depends on the interest, Academic and background for the chooser who is preparing for it. There are below 7 most popular and most usable modules.
(1)MM (SAP Materials Management)
(2)HCM (SAP Human Capital Management)
(3)CO (SAP Controlling)
(4)FI (SAP Financial Accounting)
(5)SD (SAP Sales and Distribution)
(6)PP (SAP Production Planning)
(7)QM (SAP Quality Management)