I am a complete beginner, I need to make a midi controller with USB output. I need at least 10 faders and 8 rotary knobs. Which motherboard would you recommend to me?
And what parts would you recommend to me?
I'm not sure what you mean. An Arduino does not interface to knobs, but to the electrical component under the knobs.
With faders and rotaries do you mean potentiometers (analog inputs)?
Most Arduinos have a serial USB interface (COM port emulation). Is that what you want?
Sorry I know its hard with me 
I mean midi controler (with usb output) with potentiometers like:
What do you mean by "USB Output"?
That's the key question.
It means, just plug USB from pc to controler and control software at pc.
Via some specific protocol? USB MIDI? Serial? something else? What software is it supposed to work with? USB can do a lot of things...
If you want it to act as a USB MIDI device, that's tricky - I've heard a lot of talk about "HIDUINO" in this context - that requires an Uno or Mega with the 16u2 chip for serial interface (not CH340G or FT232 like some of the clones have).
If you want USB Serial, any board will work. If you want USB HID or something, you need Micro/Leo.
Beyond that, I don't know enough about midi or audio.
I need it with Freestyler DMX (lighting software). It should work like classic midi controller just plug and play.
Do you want it to talk DMX, or MIDI? Could you use DMX cables instead of USB? If you want plug-n-play USB MIDI then you could use a Leonardo or Pro Micro and maybe a 4051 multiplexer. Or google the Hiduino as DrAzzy suggested.
Jimmy