This often depends on the mouthpiece. Generally a mouthpiece with a wide tip opening needs softer reeds than one with a narrow opening. Some people use harder reeds (3.5 – 5) to get a louder sound, however with practice and proper use of the diaphragm it is also possible to play loud on softer reeds, with the added advantage that you will get more flexibility of tone. Wide vibrato and note bending becomes easier. I use 2.5 reeds with a 125 RPC tenor mouthpiece.