Having multiple saves at the same time is planned.
There isn’t any reason to limit the number of available saves (other than if they are big and take up a ton of disk space).
If the save files are reasonable size, I’d imagine we’ll have a similar system to most games where the player can make as many save files as they want, additional there would be quicksave and autosave slots.
If the saves are huge there might be some need to have like a “world history” files that were separate from the saves, so multiple saves could share one history file in order to save disk space.
I don’t want to have a galaxy like save select screen as in Thrive you’ll probably be able to whipe out your entire galaxy, and that should / would affect the other saves. I just dislike that spore kinda pretends that the saves are happening at the same time and have some kind of interaction, but not really being able to affect the other saves.