If your dozens of scripts are top-level (on the html itself) then you can fetch all of them in parallel — so you wait your connection speed roughly twice (roundtripped).

If they’re nested dependencies (where you don’t identify the next necessary js until you have the first one in hand), the dependency won’t start getting fetched until the predecessor is retrieved. So you wait your connection speed multiplied by depth (x2; roundtripped)

The goal of the bundler is to take the second scenario and turn it into the first scenario.

If they are not waterfalling then yeah. But when a depends on b and b depends on c you will be fetching those serially.

The bundler will have those preprocessed and bundled together.

There will definitely be an inflection point, and it's very dependent on many factors: number and size of scripts, how much of the initial content is SSRed, network bandwidth and latency etc.

Browsers are good at caching JavaScript files, so it depends if the scripts are already cached or not.