no need to make multiple accounts, who told you that? You need one account and then can contribute to all Lemmy communities on all servers.

The flaw of lemmy's design is that communities reside on a single service, so you will end up with multiple "Science" communities, because in the founding phase several people created such a community of the same name on multiple servers. They should instead have used a hashtag-based approach (granted, problem then is the tight-knit moderation that makes subreddits like AskHistorians so valuable), or implement something like communities subscribing to other communities (like a community-ring) etc.