Ask HN: Team size thresholds for various web app architectures

I have been involved in early stage teams where on day one it's decided to have a seperate frontend app ( usually React ) with a seperate API / Backend ( usually Rails ).

When it comes to team size, if you're building a team and a SaaS app, what size team would you want a generic SaaS app as a Monolith vs team size for SPA + API backend? What impact would available team size have on architecture decisions? E.g. a team of 2-3 vs a team of 12 vs 30.

I think the Google/Facebook/AirBnB/etc way of doing things isn't suitable for a lot of small teams, however it is the latest hot tech that always seems to get implemented in early stage startups. How should available team size impact technology choices? Any rules of thumb here?