Doesn't say anywhere in my textbook lol
It is just a number that depends on the object and the fluid it is travelling through.
A javelin travelling through air would have a lower value of k than a car travelling through the same air.
A javelin travelling through air would have a lower value of k than a javelin travelling through water.
But you don't have to worry how this would be calculated/measured. Either:
(1) It will be given to you
(2) You use k intact without substituting
(3) Its value can easily be worked out by giving you the resistance for a particular velocity
The only confusion is that k is used by different books to represent two slightly different things:
(1) The proportionality constant in the Force-velocity equation (so the constant in the Acc-Vel equation is k/m)
(2) The proportionality constant in the Acceleration-velocity equation (so the constant in the Force-Vel equation is mk)