Be careful of people that (a)seem to give too many different people the same type and (b) seem to always have conflicts with people of that type. They may be people who are in need of mental health treatment but are projecting their problems onto others by scapegoating a given type.
Generally, the pathology works as follows: they identify a certain type that they consider a problem. (Six, INFJ, etc.) If you really get to know them, you may find out this is the same type that they believe a parent figure to have. They will usually have a past history of conflict with this type. They also will assign a statistically improbable number of people with this type. Then, when there is a conflict, they will cite the person's type as the cause of the conflict.
I used to know someone that did this. He believed his mother was a Type Six. He then identified many of his exes, former friends and adversaries as this type. He also seemed to know a disproportionate amount of supposed Sixes: his boss, his friend's roommate, his best friend, a number of women he had befriended online, a co-worker…you get the idea. As I got to know him better, I learned that he was rather unstable: he had physically assaulted one of his girlfriends, cheated on another, abused cocaine, flew into rages and even brought a lawsuit against a friend's roommate who had forced him to leave the premises.
The thing with personality disorders is that there's a tendency to believe one's own behavior is normal and that others are the problem. (This may be truer for personality disorders like Narcissistic and Antisocial than for disorders like Borderline or Avoidant where the extent of internal suffering may lead the person to seek help.) The enneagram especially is attractive for that reason. Partly because the type descriptions focus on the negative, but also because it gives one a lot of ammunition when in conflict with others. It's a great system, and while I don't think most people misuse it in that way, it is an unfortunate (if less common) side effect.
Of course, not everyone who over-diagnoses a certain type is pathological. Most of the time, it's due to a lack of critical thinking. Sometimes, too, people don't distinguish between the general energy of the triads and the primary types. i.e. you might pick up on the fundamental gut energy and type a lot of people as nines who are eights or ones and so forth. Where you have to be careful is when the person also disparages that type and seems to have a lot of conflicts with people of that type.
If you see someone that is assigning a given type to too many people, ask them their parents' types. If one of their parents' types is also the type they over-type in others, then take what they say with a grain of salt.
Also, bear in mind that no one affiliated with the enneagram has done extensive research to determine which types are "most common." (The MBTI people have done this, which is why they speak of the relative rarity of the types.) So, when you hear someone say that a given type is "common" realize that a lot of confirmation bias has likely played into this determination.
