How much documentation does an early stage startup really need?

You do not need a heavy documentation process, but you do need a few key artifacts that stay up to date. Start with a concise architecture overview, a setup guide, and a clear description of your main user flows. Capture important decisions in lightweight design notes so future you can understand why something was done a certain way. Encourage engineers to document interfaces and contracts that other people depend on, such as APIs and data models. Good documentation is about reducing friction, not creating paperwork, and it can be built gradually as questions repeat.