It's a number which is manageable and watchable -- there's not much point in "watch list" filled with people you don't have the capability to watch.
And this is where the public eye needs to be, for the sake of having a voice. In the next 12 months it is highly likely work will take place to "update" the SIS legislation, likely with greater powers, and moves to increase the powers of the Government Communications Security Bureau (GCSB). The briefing to the spies' Minister, recently released, said as much.
The reason the public need to have a close eye is linked to the spies' dilemma. If you can't watch everyone, how do you know you are safe? How many do you watch? And does it make a difference?
The finite powers of the intelligence agencies means there will always be difficult decision-making. Who goes on the list and who comes off? It's risk management on a national scale, because getting it wrong has the potential for catastrophic consequences.
The watch list is a fluid, shifting and troublesome management task. Names appear and disappear on it depending on the actions of known individuals, and the emergence of new actors who were previously unknown. It is influenced by domestic and international events. With finite capacity, does the SIS focus its attention on some wannabe jihadi with a Facebook page and a threatening message against international obligations to watch funders of terrorism abroad?
Intelligence professionals estimate it takes a team of 20 people to watch one person for 24 hours a day, seven days a week. That's eyes-on spying, which is not usual.
We make up numbers -- as do our international partners -- with surveillance warrants, rarely video warrants employing listening and watching devices and other secret squirrel tools. Emails and phonecalls are intercepted, content scrutinised. This is why the SIS is currently advertising for a Somali linguist -- foundation skills are "discretion, discretion & more discretion", according to the job advert.
Relationship management is important to the SIS, too. They have regular contact with leaders in the Islamic community and have people who will drop in on mosques. And there's police support, too. The expertise police hold in obtaining human intelligence through surveillance squads expert at fitting in, often to the communities from which they came, is an important force multiplier.
But it comes back to being a numbers game. In June 2002, for the year during which the September 11 attacks happened, the SIS had 111 staff. By June 2007, the number had increased to 183 and by the middle of last year the number was 225 staff.
Don't forget the SIS has other jobs - physical security of Government fixtures, security vetting of public services staff. They're not all out there watching those they believe need to be watched.
In October 2014 the threat level was upgraded from "very low" to "low". The next month, an extra $7 million funding was found for the service. SIS director Rebecca Kitteridge told the Herald in January the number of staff was linked to the threat level. The extra funding led to a period of "very active recruitment", she said.
More spies means more capability, means more watching, means a watch list which is able to grow beyond its current constraints. If we can watch another 40 people to make 80 people, does that mean we still have an additional 40 in the shadows, as they are now?
It only takes a sliver of fear to imagine the bad things which lie in the dark.
More spies, more legal authority - it all gives the security services the opportunity to shine a brighter light.
But, for all that, outside the light will always be dark. Again the question - if you can't watch everyone, how do you know you are safe? You don't. And you always know there is more which could have been done - but only after it has all gone wrong.
Consider this answer from Michael Morrell, who led the CIA for a while. He told Politico magazine that he believed the US National Security Agency (NSA) leaker Edward Snowden had blood on his hands over the Paris attacks.
Politico: "But absent the Snowden disclosures, if all these methods had not been exposed, do you think that US intelligence would have detected the plotting that led to the Paris attacks?"
Morell: "Don't know. But it certainly would have given us a fighting chance."
He doesn't know and no one will never know, not for sure.
What we do know is, as of today, there is no record of any terror attacks stopped by the extraordinary capabilities of the NSA, despite the massive expansion of capabilities after September 11. The record appears the same in all Five Eyes countries: Australia, Canada, New Zealand or the United Kingdom.
In March this year, Snowden talked of the increase in surveillance, saying: "They're not going to stop the next attacks either... they're not public safety programs. They're spying programs.
"But the question that we as a society have to ask [is], are our collective rights worth a small advantage in our ability to spy."
This is the question the public will be asked next year around the expansion of New Zealand spy laws.
For Western governments, the possibility of what lurks in the dark is magnified by a political lens which some feel explodes risk on a scale which could be disproportionate.
For the West, it seems no risk is publicly acceptable and yet, true freedom has always been about accepting risk.
