There are 56 unique target awards in the game. You can track which city targets you've smashed through and how many times you've smashed through them from the main menu by selecting View Progress> Award Browse and the city target awards are the first half of the collection.
You're more than likely to hit every city target by the time you have finished the Shatter Bay Career but this is assuming you don't always use frag immediately as it's available to you or to frag other racers when not necessary.
Often using city target shortcuts will gain you places and points in the race. If at the end of the race you happen to miss one or two city targets, you can always replay the event and destroy any targets you missed last time although this isn't always necessary.