87 adventures per year means one adventure every four days, which is definitely impossible. For one thing, a large percentage of those adventures take considerably more than four days apiece, with many covering weeks or more. "The Paradise Syndrome" alone spans two months. "Mind-sifter" in Bantam's The New Voyages
spans over a year.
For another thing, travel time has to be taken into account. Space is enormous and one should logically expect weeks to pass between consecutive adventures. Especially if characters are injured or the ship is damaged at the end of one mission but they're back to normal by the next. (In "Tomorrow is Yesterday," Spock estimated it would take at least three weeks at a starbase to undo the Cygnetians' reprogramming of the computer.)
Not to mention that it's unrealistic to expect them to go from life-threatening adventure to life-threatening adventure with no downtime. Sure, you could assume that Starfleet tended to assign them specifically to high-risk situations, but a lot of TOS stories start out with routine star mapping or planetary surveys or delivery runs. It stands to reason that they would've had a significant number of routine, uneventful missions in between the dramatic, dangerous ones.
So really, even fitting 79 adventures into 3 years -- or 101 adventures into 4 years, counting TAS -- is pushing it. That's just 25-26 adventures per year, meaning about one every two weeks on average. Given travel, repair, and recovery time between televised missions, and given the duration of the missions themselves, even that would leave very little room for extra adventures in between. Fitting in one adventure every four days? Completely out of the question. (Not to mention how many TOS novels and stories contradict one another anyway.)