One circuit may be the optimal way to go, but some who have really powerful amplifiers may want more power than 15 (or even 20) amps for the whole system.
Also, some don't care for conditioners/filters and would rather plug their gear directly into the wall, particularly analog gear. If installing new power, having multiple lines should not be a problem if they are dedicated (i.e., directly wired to a single outlet so they do not power anything else) and all run from the same side of the panel. I believe there may be other best practices such as keeping audio circuits away from circuits powering noisy fans and resistive/variable type devices. Another option is to establish a sub-panel (like 50 amps) and then run all the audio system outlets from that. I am fortunate that my 3, dedicated, 20A lines are absolutely quiet.
Also, some don't care for conditioners/filters and would rather plug their gear directly into the wall, particularly analog gear. If installing new power, having multiple lines should not be a problem if they are dedicated (i.e., directly wired to a single outlet so they do not power anything else) and all run from the same side of the panel. I believe there may be other best practices such as keeping audio circuits away from circuits powering noisy fans and resistive/variable type devices. Another option is to establish a sub-panel (like 50 amps) and then run all the audio system outlets from that. I am fortunate that my 3, dedicated, 20A lines are absolutely quiet.