Do I need to measure impedance before every CAN termination?
Not at each device. You're supposed to daisy chain CAN devices so you only need termination at both far ends, preferably a little bit past the last device (rather than slightly in front of it) so the termination is truly at the end.

Do you know how if you pour water into a bucket it splashes? And the faster you pour the more it splashes? Now imagine instead of a bucket it's a big long pipe with a closed end.Why does it splash? Because the water flow abruptly changes due to the bottom of the bucket or pipe. Now instead of a closed end the pipe diameter abruptly changes and narrows.

The water is like electricity, The pipe is like a conductor, the pipe diameter is the characteristic impedance, and the splashes are voltage reflections (and technically the far end of the pipe communicating to the near-end of the pipe that something has changed so the system can react and reach equilibrium).

Pouring faster is like trying to have a faster transition edge. How do you stop the splashing? One way is maybe to make a hole at the far end so that the water flow doesn't abruptly change. That's what a termination basically is (there's more to it and in electricity there's other ways to go about it that don't have mechanical analogs).

It's characteristic impedance that ideally determines the termination values you want, not impedance.

Go read up on transmission lines. Also:

Why does the CAN bus use a 120 ohm resistor as the terminating resistor and not any other value?