When A Founder Should Hire Someone Else To Run The Company
The question is not competence. It is whether the job the company now has is the job you want.

The decision to bring in an outside chief executive is usually framed as a judgement on the founder's ability. That framing produces bad decisions, because it makes the question about ego when it is really about fit between a person and a job that has changed.
The job changes
The role of running a company of fifteen people and one of five hundred share a title and very little else. The first is inventing a product and selling it personally. The second is building the systems through which several hundred people make decisions you never see.
Some founders make that transition well and enjoy it. Others are excellent at the first job, unhappy in the second, and entirely capable of recognizing the difference.
Honest signals
The useful signals are internal. Are you spending your time on work only you can do, or on work you have not delegated? Do you dread the calendar? Are decisions waiting on you because you have not built anyone able to make them?
The last one is the serious signal, because it compounds. An organization that cannot decide without the founder does not scale regardless of the founder's talent.
The alternatives
Between doing everything and stepping aside there is a wide middle. Founders move to product, to technology, or to a role focused entirely on customers, and hire operators for the rest.
A founder who becomes chief product officer in their own company, with a chief executive running operations, is a common and frequently excellent structure. It requires clarity about who decides what and genuine willingness to be overruled.
What goes wrong
The failures are almost always about authority rather than ability. A chief executive hired to run the company, with a founder who retains board control and continues to direct staff informally, has been set up to fail.
If you do this, define the decision rights in writing, tell the organization clearly, and then hold to it in the meetings where it is hardest.