7 dominent chords?

When dealing with chord progression and the tonic, sub dominant and domient chords, ie of C maj, u have the chord C, F, G7, why is the dominent a seven?

Because that is how you get the dominant seventh chord. The key of C major is C D E F G A B C. If you build a triad with a added third over G (dominant), you get G B D F. The spelling of a dominant seventh chord, 1, 3, 5, b7. It's the only chord in that key that can be a natural dominant seventh chord, and because of this it strengthens the sense of key by it's relationship to the tonic. G7-Cmaj produces a stronger sense of key than Gmaj-Cmaj. Why? Because G7-Cmaj only exists within one key, C major. However Gmaj-Cmaj exists in other keys, ie I-IV in G major. So basically you could say that by having the dominant 7th chord, your securing the sense of key.

Here's another way of looking at it.

C maj scale is C D E F G A B

You build chords by choosing every other note, in this case starting with G.

G B D F

This is sometimes referred to as "stacking thirds". I learned theory from jazz/blues players, so my description might be different than what is given in some theory classes. Hopefully this helps you understand.

F is a whole step, or two frets below G, and B is a maj 3rd above the root, G. The chord with the maj 3rd and the b7 is a dominant 7th chord.

Here's the same thing from the A maj scale

A B C# D E F# G#

The 5th note is E, take every other note to build the chord

E G# B D

Again, this chord has a maj3rd (G#) and a b7 (D) in relation to the E note. It is a dom7th chord.

If you build a chord from every note in the scale, you get the following chords (using roman numbers instead of names)

Imaj7 IImin7 IIImin7 IVmaj7 Vdom7 VImin7 VIImi7b5 (sometimes called half diminished).

From the C major scale, these would be:

Cmaj7 Dmin7 Emin7 Fmaj7 G7 Amin7 Bmin7b5

Peace,
-Rick