Do you get the TSA pre check mark for each flight no matter what? Or is this flight specific?
It is indeed specific to each individual flight, though it will usually be on each boarding pass for a particular part of your trip (i.e. all of your outbound boarding passes or all of your return boarding passes.) You won't know for sure whether you'll get it for a particular part of your trip until you check-in and see your boarding pass. Granted, in most cases, you'll only need to clear security once per direction anyway, but sometimes changing terminals can require clearing security again on a layover (e.g. at LAX or SFO.)
According to TSA, there is no guarantee that you will have it on any given flight, even if you have a KTN from TSA or CBP. I have seen many instances personally in which I or someone I was traveling with had it for just the outbound flights and not the return or the other way around.
TSA incorporates unpredictable security measures throughout the airport and no passenger is guaranteed expedited screening.