It's really up to the creator to decide exactly how "rare" traits are, but keeping track of which designs have what is pretty simple with a masterlist, which can then help keep the rarer traits "rare" so you don't have an overabundance of them
Do creators often have a specific percentage in mind for "common" versus "rare"? Do you count how many of your designs have each trait?
Yeah, I'm just wondering if any creators have a set percentage of how many they'll make with each trait, or if they just play it by ear.
A lot of people just play it by ear, although I know at least one creator uses an actual physical d20 to roll for traits when they make adopts or sometimes to determine what rarity a custom is (they state this in their info, which is the only reason I know) in addition to having a limit on the number of rarer adopts they produce per year. It seems less common to do it that way, though.
e: i do kind of a combination where I try to keep a rough ratio and if it gets too high in one area I try to release more commons to compensate. Very loose, though.
honestly I just play it by ear like clyncyerudje mentioned!! my trick is to make rare traits either vastly varied so that its actually difficult to reuse all the same traits again, or make the rare traits something that looks really cool/makes sense for the species, but i don't like to draw that much-//HIT