Well one thing you do bare in mind is when Plutarch was writing. Now he's generally a good source but in Plutarch was living in the 1st and 2nd century CE, the golden age of Sparta was several centuries gone while most of the good sources on sparta during its real age of dominance most of the good sources of Sparta were people like Thucydides not Spartans themselves, you'll notice in the relevant section he's citing Aristotle, Plato and Thucydides. By Plutarch's day the main thing spartans had been doing for several centuries was sitting around making the occasional pithy remark and talking about how tough they used to be. The legend and self image of Sparta was everything them.

But onto the point itself lets look at the actual passage on the subject.

Now in all this there is no trace of injustice or arrogance, which some attribute to the laws of Lycurgus, declaring them efficacious in producing valour, but defective in producing righteousness. The so‑called "krupteia," or secret service, of the Spartans, if this be really one of the institutions of Lycurgus, as Aristotle says it was, may have given Plato also this opinion of the man and his civil polity. This secret service was of the following nature. The magistrates from time to time sent out into the country at large the most discreet of the young warriors, equipped only with daggers and such supplies as were necessary. In the day time they scattered into obscure and out of the way places, where they hid themselves and lay quiet; but in the night they came down into the highways and killed every Helot whom they caught. Oftentimes, too, they actually traversed the fields where Helots were working and slew the sturdiest and best of them. So, too, Thucydides, in his history of the Peloponnesian war, tells us that the Helots who had been judged by the Spartans to be superior in bravery, set wreaths upon their heads in token of their emancipation, and visited the temples of the gods in procession, but a little afterwards all disappeared, more than two thousand of them, in such a way that no man was able to say, either then or afterwards, how they came by their deaths. And Aristotle in particular says also that the ephors, as soon as they came into office, made formal declaration of war upon the Helots, in order that there might be no impiety in slaying them.

While the bit of the bottom suggests yes once a year the Spartan leaders made a formal declaration of war against their helots which proved a kind of legal pretext for repressive actions against the helots. And we see at various points that various spartans murdered various Helots. But I don't really see any indication that every single year every spartan killed at least one helot. Many historians believe the Krupteia/Crypteia mentioned here were some kind of secrete police, and that while the spartans might have believed making sure some of their warriors killed people while they were still young was of some value its likely that they were deployed to actually kill any significant number of helots where and on years when there was some level of unrest and they were directing this killings at people or small communities that contained dissidence as a form of terrorism. While we can't really know how many people died in that you don't have to make that much of a dent in a population in demographically terms to have a serious effect on a subjugated people in terms of repressing them.

Likewise the line about picking out the strongest and bravest helots and killing them. That's a reference to this in The History of the Peloponnesian War

The Helots were invited by a proclamation to pick out those of their number who claimed to have most distinguished themselves against the enemy, in order that they might receive their freedom; the object being to test them, as it was thought that the first to claim their freedom would be the most high-spirited and the most apt to rebel. As many as two thousand were selected accordingly, who crowned themselves and went round the temples, rejoicing in their new freedom. The Spartans, however, soon afterwards did away with them, and no one ever knew how each of them perished.

Again the Spartans aren't doing this randomly, they're at a time when they have some very serious concerns about a revolt from the Helots, they've recently used this group they've repressed as soldiers and they're worried the Helots in their forces are about to turn around and rebel. So they pick out the natural leaders and bravest (and therefore more likely to risk rebellion) from their helots and they purge them. I'd also say while Thucydides is a good source 2000 sounds like it might have been exaggerated somewhat and for the purpose of terrorizing their helots the Spartans themselves would have a certain motivation to exaggerate the number they butchered while their enemies would have a motivation to over describe the brutality of the Spartans.

In general i think its most likely based on the evidence we have about them and more generally comparing them to the patterns done by other regimes that have a small minority repress a large majority that its likely that the peroids when the Spartans felt the least secure would be the times they were most likely to go round doing things like having the Krupteia operate on anything like a regular basis or Spartans wondering out into their fields and killing a helot that looked tough. So the times when they were most brutal are also likely to be those between like 3rd century or so and maybe the 1st century BCE when the population of actual spartans and the importance of the city had declined. So there weren't that many of them actually around to do terror killings like this and it was most likely to be specific forces assembled for the task of purging possible dissidents.