I have three monster books -- MM, Creature Collection I, and Creature Collection II.
I also have monsters floating around in dozens of other supplements that are not specifically monster-based.
So far, I have found this to be way too many. But I also know that, amongst GMs, I am in the minority on this one.

I have a handful of "workhorse monsters" -- skeletons, zombies, goblins, etc. Pretty common types, since I figure they make up the majority of enemy critters in any given world. A lot of the others are "one-offs" -- a creature that appears a single time. Then again, I'd say around 50% of "monster" encounters are NPCs. I like the idea of character classes vs. character classes.
As to the question of "monster-or-scenario-first", for me, about 80% of the time, it is scenario first, with monsters added in as appropriate to the scenario I am trying to create. About one adventure in five, however, is based around a specific type of monster: "Gee, I'd really like to see what the group could do against a well-defended vampire and its minions."