Don't hire a role you think you need until you're sure you need it. Sometimes startups think "we need an HR person" or "we need a marketing person" before those jobs are actually at the point where they require a full-time person.
But after your first few engineering hires, you will probably know well whether you need, say, a backend engineer. You will have people doing some of that work, and be able to look at your roadmap and estimate correctly.
But for first-of-their-type roles (like my marketing or HR examples), that's harder - often part of it is startup leadership thinking "we could be doing so much XYZ I don't know about", instead of "we're doing 10 hours of XYZ a week and I know we need 40".
Once you've decided you need the hire, you want to get a person as smart as possible.