So, is strange that Ardour only see 2 inputs...

Can you send a link to the plugin that you are testing?

I think at some point Paul said this is indeed the way things are. But you can see for yourself.
http://www.sirlab.de/linux/download_vocoder.html

$ analyseplugin ~/local/lib/ladspa/vocoder.so

Plugin Name: "Vocoder"
Plugin Label: "vocoder"
Plugin Unique ID: 1441
Maker: "Achim Settelmeier (adapted to LADSPA by Josh Green)"
Copyright: "None"
Must Run Real-Time: No
Has activate() Function: Yes
Has deativate() Function: No
Has run_adding() Function: No
Environment: Normal or Hard Real-Time
Ports:  "Formant" input, audio
        "Carrier" input, audio
        "Output" output, audio
        "Number of bands" input, control, 1 to 16, integer
        "Band 1 Level" input, control, 0 to 1
        "Band 2 Level" input, control, 0 to 1
        "Band 3 Level" input, control, 0 to 1
        "Band 4 Level" input, control, 0 to 1
        "Band 5 Level" input, control, 0 to 1
        "Band 6 Level" input, control, 0 to 1
        "Band 7 Level" input, control, 0 to 1
        "Band 8 Level" input, control, 0 to 1
        "Band 9 Level" input, control, 0 to 1
        "Band 10 Level" input, control, 0 to 1
        "Band 11 Level" input, control, 0 to 1
        "Band 12 Level" input, control, 0 to 1
        "Band 13 Level" input, control, 0 to 1
        "Band 14 Level" input, control, 0 to 1
        "Band 15 Level" input, control, 0 to 1
        "Band 16 Level" input, control, 0 to 1

--
Louigi Verona
http://www.louigiverona.ru/