Well, a quick rule-of-thumb is that if we were to spread the 850,000 articles on en: out over the 600-or-so admins, that would make for 1400-or-so items to be added to each watchlist.
If the # of regular RC watchers is, say, 5x the number of admins, that would become a more reasonable 280.
I recently purged my watchlist down to 2200 items. It's back up to 2300 a few days later. Of course, it's the same hundred or so articles that keep popping up, a smaller subset that keep getting vandalised. I understand why Evolution gets vandalised constantly. I am clueless as to why Deforestation gets vandalised all the time.
A few hundred extra articles wouldn't be a big deal.
Ian (Guettarda)