The terraforming candidate anomaly is actually quite rare. But it can be repeated.Too few? They're the most spammy and common anomaly in the game.
Most anomalies can only fire once.
So, if the galaxy is large and you get to explore much of it, you'll see many terraforming candidates in the end. However, there will be nearly none pf the, before the other anomalies run out. It is noticeable, when big areas only become accessible after jump drives, suddenly there is a cluster of candidates.