Hello to all!
I did the design of a new controller and I would like to create all the intern electronic device to control a software. The controller includes 7 pads, 8 knobs and a small monitor to make quick adjustments without looking at his computer.
Which Arduino card I need to program all this device? It's the same that I put inside the controller?
What is the most small and powerful monitor to do it?
I'm a really beginner so thanks in advance for you help!
did the design of a new controller...
So, I assume that's a design concept? With the actual electronic, software, and mechanical design remaining to be done? 
You want to make a MIDI "control surface" something like [u]this[/u], right?
There's a [u]MIDI Library[/u] and some examples of how to interface MIDI. You might want to get an Arduino UNO, build a MIDI interface, and make it do some simple control totally in software (with no knobs or switches) before jumping-in and trying to design & build the whole thing.
I don't know if there are any USB-MIDI examples, or if your computer will need a traditional MIDI interface... I have a feeling, you'll need a regular-traditional MIDI interface.
MIDI commands are fairly simple, but "video" can take lots of memory. There is an [u]LCD Library[/u], but if you need lots of resolution, or a color display, or to display lots of graphics, etc., the Arduino may not be up to the task.
If you want "real video", look into the [u]Raspberry Pi[/u], which is sort-of a "competitor" of the Arduino.
The controller includes 7 pads, 8 knobs
The Arduino Uno has 14 I/O pins, although they can be "expanded" using multiplexing.* The Mega has 54 I/O pins and more memory, so it might be a better choice.
First of all thanks to your answer. Yes I did the external design of the product and I'm also developing new uses to play on "Traktor scratch Pro 2" From NI. It's a personal project and I do also some research to know how create the code program.
I'm working on OSX and the reference controller is the "Kontrol X1" from NI. DJ Controllers : Traktor Kontrol X1 | Traktor So one goal is also to do and interior electronic design with the less of pieces. I'm looking to all the more little electronic things to do it.
The control panel will show writing and figures from the software to see selecting choice in real time. In use, turn knobs and press knobs will create new writing. It's also possible that I put changing colors for pad. So do you think the Mega is still enough or the Raspberry Pi is better ? May be just a Uno with multiplexing surface is enough to install all the command where I need ?
It's also possible that I put changing colors for pad. So do you think the Mega is still enough or the Raspberry Pi is better ? May be just a Uno with multiplexing surface is enough to install all the command where I need ?
I've never built anything like that and I would only be guessing. It's a fairly advanced project for a beginner. You might consider just adding a 2nd small monitor to your existing set-up, or building a display to go-along with the Trackor controller. I wouldn't consider building a display to work with the Trackor to be a beginner project either, but it's easier than building the whole thing.
Maybe you should start be deciding what kind of display you want to use and how much resolution you need. The number of pixels is a starting-point for how much memory you'll need. Then, you'll need some memory to keep the display updated and to hold your code and other variables. But I think the video monitor it the most demanding part of your project.
The Raspberry PI has composite video & HDMI built-in. (I've never used the Raspberry PI.) The Arduino doesn't have any video/graphics built-in, so it usually communicates serially with an LCD display.
On the other hand, the Arduino has more general-purpose I/O pins and analog inputs (especially the MEGA) for knobs & controls.