You won’t be able to contribute to a project on any of those instances, unless you register on it. So if you are a prolific contributor, you might end up signing up on dozens of those instances.
Forgefed is a federation protocol for such instances. It’s based on ActivityPub - the same that powers Lemmy and Mastodon. You can have just one account on a single instance and still be able to contribute to projects on multiple others. It’s still in the works though. It’s expected that at least gitlab, gitea and forgejo will support it.