In my personal view, in a developed "Lemmyverse" one instance would cover a topic about as specific as a general subreddit.
When making an instance I first thought about making a much more focused instance. To give you an example, one idea was to focus on reptiles and amphibians.The communities would then be much more specific - salamanders, pets, geckos, snakes, etc... And this structure can certainly work, as there are many forums like that! For example, this forum about microscopy has a healthy community. But several people with very specific interests would need to show up simultaneously to build an engaging community. So I decided to broaden the topic to cover science and nature in general - and at the moment it is fine because there are not that many users.
However, once Lemmy becomes more popular and hopefully scales up successfully, there will be many more people with specific interests looking at these sites. Then it should be possible to form reasonably engaged communities based around niche topics. At that point, an instance dedicated to "Science" would be way too broad!
It is great that many users are joining in by discovering the instances that already exist. Hopefully many will realize that the really interesting part is the ability of creating and self-hosting an instance in a server that you have control of. I think that the best way of scaling up would be by having lots of people hosting small instances.