[ksec]

>I am trying to figure out if I'm capable enough to be a CTO..... but my question is more about how much I should already know on day one.

It depends on the type of Start up, is it an SaaS, or ML / AI based company.

You will actually have to understand more about your business goals, than any other programmers below your organisation.

You should say "No" to 99.9999999% of suggestions using the latest and most hyped technology.

Your choice of Technology is likely only there to achieve your business goals, not for your inner child or colleagues to have a playground and mark another thing on their CV / resume.

You will also have to set reasonable schedules and timeline. And be able to deliver them on time.

And here is another unpopular opinion.

Your biggest fight may likely not be your subordinate but with your co-founder / CEO.