Hello Silvain,
Note that I specified \voice number, so number is not part of the name, but an
argument. But Lilypond does (similar to TeX) interpret \command1 as \command 1
(which is the reason why numbers cannot be part of variable names, also
similar to how \commandA\commandB do not need to be separated, as well as
\command{ ... } is possible).
This is the exact reason why I am suggesting this option: For instead of
having a fixed number of commands \voiceOne, \voiceTwo, ... you simply have to
say \voice1 (\voice 1), \voice2 and you can thus easily use an arbitrary
number of voices.
Cheers,
Valentin