There is this belief in the Enneagram community that the distribution of Enneagram types is far from equal. That's not my experience, but I have a theory why - apart from ego interests that I don't want to assume here - people might think that:

The most spreaded belief of this kind that I saw so far, is that type 4 is the rarest type. I think it's an illusion. 4s have an intrinsic motivation to be unique, and that makes them behave differently. Not only from the "normal people", but also different from other 4s. The result is that the spectrum of the 4s' behavior is probably the most diverse of all 9 types. This might make it difficult for 4s to recognize other 4s as the same type, especially if they think of Enneagram types as the behavior categories they are not. So, 4s have the rarest behaviors, but type 4 is not actually rare. Similar with 8s, that tend to be perceived as rare because of being drawn to the extremes.

The same people often say that types like 6s or 9s are the most common. Here we have the opposite effect: 6s and 9s are both part of the so-called mediation triad (369) that makes people want to work as mediators between others, which makes them take mixtures of the behaviors of other people. Result of this is, that 6s and 9s tend to have a lot of behaviors that are more common than their type, but that are associated with their type. This produces the illusion that the types 6 and 9 are more common than they actually are.

I can only guess about the distribution, but in my observation, it is pretty equal. There are certain cultures that are dominated by a certain type, but those dominant types can be all the different types. France is said to have a 4ish culture.