Partial reply – all of the clues appear to level to the quantity:

Seventeen

The first and the final of the times of the week

…are numbered 1 and seven, respectively

I and Z in Latin and Greek:

I is 1 in Roman numerals, and Z is 7 in fashionable Greek

In N and in P, and interval and group:

Looking on the periodic desk of parts, Nitrogen (N) is in interval 2, whereas Phosphorous (P) is in group 15. 2 + 15 = 17

Of them I consist and I at all times have been:

If this assertion particularly refers back to the first half of this sentence, “I” may very well be taken to imply Iodine, which is in group 17 of the periodic desk.

But now in my identify, what quantity is seen?

Your username is “David”, so I believe we’re in search of numbers in there.

But that is so far as I can determine in the meanwhile, since:

“D” might confer with the Roman numeral (500) or the Greek (4), “V” might confer with Roman numeral (5) or Vanadium (group 5, interval 4), “I” might confer with the Roman numeral (1) or Iodine (17) and so on.