Here is the full list of all 22 CLANNAD achievements. It takes around 50-60 hours to unlock all of the achievements on Windows.
Complete all scenarios
Complete the true ending
Achieve a 64 hit combo with Tomoyo
Jet Saito makes his appearance
Obtain all light orbs
Complete Sunohara's ending
Complete the gamebook, "Welcome to the Heartbeat☆Academy"
Achieve glorious victory in the baseball scenario!
Complete Nagisa Furukawa's scenario
Complete Toshio Koumura's scenario
Complete Fuko Ibuki's scenario
Change class to Fuko Master
Complete Ryou Fujibayashi's scenario
Complete Kotomi Ichinose's scenario
Complete the Sunoharas' scenario
Complete Kappei Hiiragi's scenario
Complete Kyou Fujibayashi's scenario
Complete Yukine Miyazawa's scenario
Complete Tomoyo Sakagami's scenario
Obtain a light orb
Complete Misae Sagara's scenario
Complete the prologue