The script is by design made for channel #1 only, I don't think it's possible to make it otherwise.
I know there is another video from us that relies on MIDI channel but actually with Live you don't need to change the transmitted MIDI channel.
Thanks for your answer, Robert. But what if i have multiple tracks, if i'm playing one of them then another, and i want to tweak the sound ? If i want to play different plugins one after another, i have to put them on different channels, no ? Thanks.
When you are using a remote control script, such as the one Arturia provides for Ableton, you do not need to use different midi channels, no. The whole point of it is that it is dynamic. You are controlling the selected device with your hardware, using the same knobs. There is no reason to use a different midi channel unless you have a massive manual mapping template and need more than the 128 offered for a single channel. But when using a remote controller script like this one, it's only going to use one channel because those few knobs control any selected device, no matter what track it's on. Just keep everything on channel 1.