How many chords can you incorporate ?
 

In a C Major Scale, you have chords such as I, ii, iii, IV, V, vi, dim vii. Now what can you incorporate with these chords that are not in C Major Scale. (And is there any terms or conditions that these new chords can be used ?):dj:

C major is relative to A minor.

You can also follow the modes. D dorian, E Phrygian, F Lydian, G Myxolidian, A Aeolian, and B Locrian.

Does that mean I can use the chords for A Minor ? What keys are in the scale of A minor anyway ?

You could solo over a C major progression with a scale in the key of Am. I do it and it works just fine. I believe the Am scale goes Am, Bdim, c major, D minor, E minor, f major and g major

Aren't the chords in a-minor exactly identical to those in C-major?

Yes, but A is the tonic chord (I), instead of C.

So in C Major, you have: C, Dm, Em, F, G, Am, Bdim, C
And in A minor, you have: Am, Bdim, C, Dm, Em, F, G, Am

Neither of these keys have sharps or flats in the key signature, which makes them relative minor keys.

there are also substituted chords and passing chords

i can probably put one chord for each word in each line in one of my songs