Great question!
First off – if the series is episodic (like Nancy Drew or The Hardy Boys), go ahead and publish the first book! Since episodic series are tied together by the same characters or settings, they don’t need to be planned out to the last volume, especially since they are indefinite. They go on until the author is done! The same with series that are semi-episodic, like the Chronicles of Narnia. The stories share characters and settings and work together to culminate in a climax that ties together the whole series, but they were written one by one because they all stand alone, and are very different in some respects.
However – if your series is one story – told over multiple volumes (Like the Lord of the Rings or Left Behind), even if you don’t write all the books before you publish the first one, they do need to be planned. Depending on your work style, this may mean a detailed outline for every volume, or a rough sketch of the overall story. How complicated the story is also plays in.
For context, almost every author writes one book at a time, and publishes one book at a time. JRR Tolkien created an incredibly complicated plot and story world, and still published one at a time. Ally Carter published the first book in her popular Gallagher Girls series not knowing exactly how the series would end. You need to take a hard look at your project and decide what it – and you – need to know before publishing book one.
Personally, I recommend outlining the entire story before finishing book 1. That way, you have the most control over foreshadowing and chiastic structure. That said, if you are a pantser, you might want to leave it a bit more vague and discover the story as you go along, though you still need to know the big twists. As the author, only you know what you need to know.
When I was working on my recently deceased WIP, I didn’t outline as much as usual (one of the many co-morbidities) and “discovered” several plot twists that forced me to go back and rewrite sooo much. If your first book is published, that option’s off the table. So make sure you’re ready.
Hope that helps!
Read, write, rewrite, repeat. It’s that easy, and it’s that hard.