Summarising from various pieces in the Campaign Setting:
The Arclords of Nex ruled Nex (the nation) at one point after Nex (the archwizard) vanished, before being driven into political exile on Jalmeray for a time. They used that as a base until they were evicted from Jalmeray by the Vudrani (the Arclords were using it as a base despite the fact that it had been gifted to the Vudrani a couple of thousand years earlier, though the Vudrani had left it untenanted for some time) and they founded one of the colleges of magic (The Arcanamirium) in Absalom. (It's unclear exactly when the Arclords were in Absalom, in relation to their other activities.)
These days they're back home in Nex grumbling about how much better everything was in the old days when Nex was at war with Geb, but otherwise aren't reported in the Campaign Setting as being upto much. There's a feat associated with them, 'Eye of the Arclord' on page 109 of the first edition Campaign Setting, which will need updating if you run a PFRPG game. (Concentration skill no longer exists so needs replacing with another skill, Spot check should be Perception, etc.)
Edit:
I don't think I've seen anything which states that they're specifically wizards, but from the feat I would surmise that at least some of them are arcane spellcasters.
I'm not sure as to their numbers. They're currently one of the political factions in Nex, indications seem to be.
