The real startup I worked at was probably around a dozen people when I joined. You could probably grow some beyond that and still be reasonably agile (that is,

The real startup I worked at was probably around a dozen people when I joined. You could probably grow some beyond that and still be reasonably agile (that is, the CEO still has a good idea of the day-to-day operations and can move the company to the right market fit), but once you're at a size where irrelevant middle management is capable of existing (probably around the point that you need two layers of management), I'd posit it's not really a startup any more. Although this might fly in the face of the common definition.