In the first few weeks of a startup’s life, the founders really need to figure out what they’re doing and why. Then they need to build a product some users really love. Only after that they should focus on growth above all else.
I think the right initial metric is “do any users love our product so much they spontaneously tell other people to use it?” Until that’s a “yes”, founders are generally better off focusing on this instead of a growth target.
(1) You can measure “do any users love our product so much they spontaneously tell other people to use it?” with net promotor score — see How to use net promotor score surveys to improve your product and Net promotor score — how to set up the survey.
(2) Cf. Is this the right goal for seed-stage startups?
(3) Re. “only after that should they focus on growth”: see The most fatal mistake to avoid as a startup.