What does the "album capacity" counter mean when I'm creating a new album?

It's simply a live count of how many albums you've used compared to your plan's allowance — shown right at the top of the create wizard, something like "Album capacity: 14 / ∞." It's informational, so you know where you stand before you add another event. On the free plan that denominator is 1, which makes the point pretty quickly that you'll want to upgrade if you're serious; on paid plans it's infinite, so the counter is really just telling you how many you currently have.