You’re injecting your own definition of Lemmy. The normal definitions would be:

• The software running on the websites and webservers. It’s the same software running on all the websites, so it’s still only one Lemmy no matter how many websites it powers.
• The subset of the Fediverse that’s accessible via Lemmy software. There’s only one of that (although defederation makes it a little complicated as you can’t access all of Lemmy from any one website).