Yes you are able to have the same machine registered twice between two different spaces. However, each machine will count towards your licenses machine cap.
Spaces are designed to be a hard boundary, built for different teams to be able to deploy to different targets independently inside the same Octopus instance.
Regarding the number of spaces available to create. Standard licenses are limited to three spaces whilst Data Centre licenses allow for unlimited spaces. The Spaces feature was primarily designed to address scalability issues customers were encountering with very large instances of Octopus.
If you have any further thoughts or questions here, don’t hesitate to let me know.