There are 30x Billboards to be found in NFS Payback.
Try to smash as many as possible when free-roaming in between story missions - this will save time later (I had 22 by the time I was finished, making clean-up way faster).
The following interactive map is very helpful:
https://nfspolska.pl/maps/need-for-speed-payback-fortun....86Filter the interactive map to billboards & chips, and filter the in-game map to the "Collectables" tab.
Do one coloured section of the map at a time to help make sure you're not missing anything as you go.