There are many answers to this question depending on the stage of the startup and where they stand in the product development cycle.
For a software startup, in the beginning they are really not doing production level coding - they are prototyping and hacking and creating testable MVP's (minimum viable products). During this phase, when the total headcount is below 5 full time equivalent, they can get away without a dedicated software quality assurance (SQA) function and share the burden of testing their software between themselves and their early users / testers.
When the team starts to get beyond 10 people and they are really starting to crank out product releases, quality and reliability becomes important because the lack of these things will result in reputation risk and an erosion of trust by customers. At this point it is prudent to start migrating towards a developer-to-SQA ratio of 3:1, moving to 2:1 as the team scales and capping off at 2:1.