So far in 2020, we've been averaging about one per month (and if you go back to the initial release in December, we've actually had 13 individual drops in three months, making the average closer to four drops per month since the series was announced), although the average is somewhat complicated by how you count themes that come with multiple drops, like the original Theros Gods during Worlds in February (technically, the Gods were five individual but related drops, although there is an argument for counting them as one big drop since buying all of the drops together was an option)
Heal the Scars was first released in the set Lorwyn, illustrated by Carl Frank
That's because persist checks the creature's existence just before it leaves the battlefield, and it still has all those counters on it at that point
The real power comes from the Lecture Hall, a three-mana Room that rewards you for dealing damage to your opponent by putting a +1/+1 counter on a creature when they deal combat damage to a player
Bitterbloom Bearer is the modern, cracked evolution of that legendary engine, putting the same oppressive upkeep trigger onto a 1/1 Faerie Rogue body
These picks ease new players into the hobby
