We don't have to chant sixteen rounds BUT "SIXTEEN GOOD ROUNDS" (Excerpts from the instructions of Srila Prabhupada)

Everyone must rise early, take bath and attend mangala-arati, chant at least sixteen good rounds, attend class, and follow the four regulative principles strictly. If these things are lax, then there is no question of spiritual life. Anyone who does not accept these things staunchly will have to fall down. 
- Srila Prabhupada's letter to Abhirama dasa, 16th January, 1975 
 
 
Please continue your work, and most of all try to rigidly follow all of the rules and regulations and chant at least sixteen good rounds daily without fail." 
- Srila Prabhupada's letter to Gupta dasa, 8th April, 1975 
 
"Please try and render devotional service very carefully following all of the rules and regulations mentioned in our books. Always chant at least sixteen good rounds daily. If you try very sincerely to do these things Krsna will help you and you will become spiritually perfect in this life." 
- Srila Prabhupada's letter to Gokularani dasi, 15th March, 1975
 
 
"Please continue these programs with added enthusiasm. They are very important. You should be very careful to follow all of the principles very rigidly. You must rise early (before 4:00 a.m.) attend the mangala- arati, attend the classes, chant at least sixteen good rounds daily, and observe the four basic principles. These things you must do without fail otherwise, there is no question of spiritual life." 
- Srila Prabhupada's letter to Caranaravinda dasa, 2nd April, 1975