If you still need it, you need to set pax_chance to 100
<Vehicle chance="50" pax="1" pax_chance="50" color="black">FBI</Vehicle> (They may come in a pair or single)
<Vehicle chance="50" pax="1" pax_chance="100" color="black">FBI</Vehicle> (Pair only)
Pax is passenger I believe
Firstly, that is not Default LSPDFR callout, I think that is assorted callout or one of them. If you want to know which pack caused that, download callout manager
most callouts have .ini files for you to disable callouts you dont want.
Use ultimatebackup https://www.bejoijo.com/post/ultimate-backup
Grand Theft Auto V\plugins\LSPDFR\UltimateBackup
Specialunits.xml
<Vehicle chance="100" pax="1" pax_chance="50" color="black">FBI</Vehicle>