How many members were in the classic Beatles lineup?

How to get close to this number

Band sizes cluster tightly, and four is the modal answer for a rock group: drums, bass, and two guitars, which covers rhythm, harmony and two melodic voices. That configuration recurs across the genre precisely because it is the smallest arrangement that produces a complete sound, and this band is where the template came from.

The word "classic" in the question is doing real work. The group existed for years before the famous lineup with different members and a different drummer, and the composition people picture is the one from 1962 onward. Band membership questions almost always need a date attached, because lineups change more often than names do.

The drummer change in the explanation is the detail worth knowing: the replacement happened just before the breakthrough, which makes it one of the most consequential personnel decisions in the history of popular music. A count of four is trivially easy; the interesting fact is which four, and when.