How many filters does a PAPR actually have, and which one do I change?

This trips people up constantly, because the box only makes a point of the filter you have to fit yourself.

The main pleated P3. This is the one you install and the one that matters. Rated P3 / TH3 P R SL, at least 99.95% efficiency under EN 12941. When people talk about changing the filter, this is what they mean.

The steel mesh pre-filter, already inside the blower. It catches coarse dust before it reaches the pleated filter. You can blow it out and put it back rather than replacing it, and doing that regularly makes the expensive filter last longer.

The activated carbon pre-filter, also pre-fitted. It reduces nuisance odours, sawdust and general shop smell. That is a comfort feature. It is not a vapour cartridge and carbon in a pre-filter does not make it one.

The routine that follows: blow the mesh out when you clean up, replace the pleated P3 when airflow drops or the alarm goes, and change the carbon when it stops cutting the smell. Use original parts. The manual is explicit that substitutes can reduce filtration, and there is no way to eyeball whether a cheaper filter meets the same standard.