How many electrons make up a charge of 3.5kc?

Well now, I don't know !
Let's figure it out.

The charge on one electron is 1.60 x 10⁻¹⁹ Coulomb .

To get 3,500 Coulombs of charge, you'd need to
go around and collect

             (3,500) / (1.60 x 10⁻¹⁹)  electrons.

That's  2.19 x 10²²  of the little fellas.

That's a big number.  But what's REALLY amazing is the
mass and weight of that many electrons:

             Mass:    1.989 x 10⁻⁵ gram

             Weight:  about  0.0000007 ounce  !

That's not one electron.  That's the whole  2.19 x 10²²  uvvum
that it takes to hold 3,500 Coulombs of charge.