We already know that Soul Hearts can stack indefinitely, even with 12 Heart containers. What happens with Eternal Hearts in the same situation? Are they still getting added between Heart containers and Soul Hearts, acting at this point as additional Soul Hearts themselves? What if I get two of those? Do they disappear, become Soul hearts or something completely different?
I tested this actually. Took a while to grind so the eternal heart card would spawn on basement 1. Everything adds up. Just because you can only see up to 12 containers (of any kind) doesnt mean that is the limit. I had managed to get up to 14 heart containers. That is, 12 full heart containers, and could still refill up to 2 empty non-displayed containers. Then I lost count after getting several Balls of Steel pills @_@
Tl;Dr: There are no limits on how many heart containers you can have, whether they are normal, soul or eternal heart containers. Only 12 first containers are shown tho.