Concern about Races/Species:
I remember a time when we were worried about an Archetype series (back then they were talking about Races) being too big. We have a small card pool and thus it is very easy for a large series to grow to become a dominant part of the card pool. In Dungeons and Dragons having more breeds of Dragon than letters in all the languages was fine but here the namesake is Elementals which are the only race that cannot be made into a creature card because they are the players.
When is a race too large for the current card pool?
IMO: 3 (for a race) or 12 (If it is a 12 element series but these should be very rare)
Basically, when does a theme stop justifying adding partially redundant cards?
Generally a 12 element series is almost near completion and any support/additional archetypes made under it are best for :
Alternate ways of obtaining the creature. (Dragon Egg, Nymph Princess and Nymph's Tears)
Minor Support to indirectly buff the cards. (Dragon Power)
Possible "Other" version of the card, or one that provides a unique alternative of it's own. (Decaying Dragon, Nymph, Void Nymph)
I think a race should probably be no bigger than 3-5 cards - some races are a bit more widespread than others.