The number of people comfortable on a boat is analogous to "how many people can a beach house comfortably sleep." Depends on the house. For boats to be comfortable, depends on the number of real berths. Many "single" berths should not count. And NEVER put people sleeping in the salon.

Most boats for coastal(island) cruising can be well managed by a couple(2 people).

If chartering, the cost is usually optimized by utilizing all the real berths (as well as the age of boat and "creature comforts" such as Gen/AC). For some it is a non-issue. For others, it is a real priority. Finding what fits you and your budget gets you to your "happy place"