There are many factors that make it difficult to answer this question. The main issues are the lack of medical recognition of the condition and the broad definition of the term midlife crisis.
The definition of a midlife crisis is also quite broad (see our article ‘What is a midlife crisis’) and so encompasses behaviors that can be found in a wide section of a population. The ‘midlife crisis’ is engrained in the psyche of the general population, with frequent references to it in pop psychology and in the media. As with any idea that is commonly believed, it becomes self-fulfilling, people see evidence to support the idea and disregard any evidence against it. For example, if someone of middle age makes a decision that appears out of character, it is easily attributed to the person having a midlife crisis. This type of casual association serves to hide the true number of sufferers, and to lump them together with a wider section of the population, some of whom may or may not be experiencing a midlife crisis.
One thing to keep in mind when thinking about how many people suffer from a midlife crisis is that if we want to understand the true number of people affected, we in the west, should stop casually attributing every seemingly poor decision, every marital breakup, any case of depression or sadness as being a result of a midlife crisis. If someone behaves out of the ordinary, it can be a result of many things, some of which are quite serious such as stress, libido issues, depression, and physical or mental illness, and not all of which are the fault, or even influenced by the person suffering from the midlife crisis.