in the film the Watcher had watched a whole bunch of Slayers. But the film mutated far from the script. There is a lack of canon about how many Slayers per Watcher, as far as I can see.
Slayers usually dont live long. Buffy didnt the first time. Kendra didnt.
I think Giles didnt expect to be watching Buffy very long. I pin that on the words 'I keep a flat in Bath'. He never totally moved to America, he kept a flat. For years. Waiting for the Slayer to die.
I have a theory but there is no actual evidence to support it. My theory is they didnt send the best Watchers to look after the active Slayer, they sent the worst. The ex rebel and the... I want to say incompetent nincompoop but I love Wesley too much. Sorta. Anyways, my theory goes, Watchers to active Slayers get killed. Buffy's first Watcher, Faith's Watcher, both killed. Even while abiding by the rules, and just Watching, not getting involved. So they wouldnt want to send the really valuable people. Send someone who can be spared. And in Giles case, someone with their very own reference library. And a back up library. Is handy because the Watchers Council didnt have to send anything extra with him. Wes didnt bring many belongings, used Giles books. So could be a rare bonus extra.
I think from available evidence the primary job of a Watcher is to train the *potential* Slayers, something neither Giles nor Wesley had done, as far as we know. A potential's training starts in childhood, possibly infancy, and goes right up to about 15 when they are called. That would mean to train one potential Slayer would take between 7 and 14 years. Only a couple of trainees in a career. Slayers are raised to be alone, seperate, so there isnt a Slayer school, just a bunch of kids with Watcher tuition. That would be pretty manpower intensive. And then what happens when they arent called?
Again a skinny on the evidence theory, but I think they get to be Watchers. The Watcher Academy that we know about is an all boys school. So where to the lady Watchers train, like the ex Watcher who went after the glove, or the lady who studied Spike, or Giles' grandmother? I think they were potential Slayers who got too old to be called. Like I said, no evidence, but I like the story dynamics it would give.
What I find interesting is that Giles got to that point in his career without having trained a Slayer at all. If theres a whole bunch of potential Slayers for every actually called Slayer, and a new one gets called every other year, that would be a ton of Watchers worldwide needed just to train potentials. So why wasnt Giles one of them?
Other Watcher duties include research, admin, and those hunter teams that came after Faith, plus RWP and his lot facing Spike back in the day. So there is more going on than just Potentials and Slayers. Giles was in research. Worked in a museum. Was that a cover story? Was it a Watcher museum? Or do Watchers all have day jobs, same way Giles was only studying the occult after school when at uni?
There is only a tiny bit of canon about the Watchers, and I've spent ages spinning stuff out from there to make a background for fanfic Giles, so I have a lot of thoughts. But the facts are way too skinny to say anything more than-
There is a Watchers Council of Great Britain
but there are Watchers in other countries too
There is an all boys Watchers Academy in southern Hampshire, England
Some Watchers do field work
some do research
some train potentials
one at a time is Watcher to the active Slayer
Potentials are trained from a very early age, but not in an academy.
Slayers dont usually last long
Potentials and Slayers work with only one Watcher
there is a handbook for how to train Slayers. Giles ignored it.
Being a Watcher is a family business- Giles, his father, his grandmother; Wesley, his father.
Giles was told he was going to be a Watcher when he was 10. Not given a choice.
Wesley knew about resurrection scrolls before that.
Their childhood and educational experiences seems to be a bit different.
Watchers can get fired. They can also get accepted back.
I can back all those statements up with quotes. It doesnt add up to much.