So, methinks I've become a bit obsessed with Sentinels of the Multiverse lately. Local game shop had a blue moon sale (as in, they always have a sale when there's going to be a blue moon), so I got the core set for about 25% off. And since then, I've bought all of the boxed expansions (two "Mega Expansions" and four regular expansions in two "Double Expansion" boxes), save the one coming out this summer, as well as a handful of "mini-expansions" (read: clamshells with a single Hero, Villain or Environment). So, I've got 25 heroes, 22 regular villains, 15 villains balanced to be teamed together and 19 environments to fight the villains in.
The core set comes with 10 heroes, 4 villains and 4 environments.