Probably nothing in the first year. And possibly never.
I'll likely get PS5 a bit later on when some games I'm interested in are out. At that point there may be a bigger SSD size option built into the PS5 for a higher price tier. Like how I got my PS4 with a 1TB HDD instead of 500 GB.
On the other hand, since that SSD is so expensive, the size increase may not be double this time around.
I'll see how that pans out, and the typical game file size for next gen games.
But if I can avoid it, I probably won't buy an external HDD to store games when I can just re-download them while I do other things.
I rarely replay games I'm done with anyway. And we'll be able to install, or un-instal, specific portions of a game. So if I'm done with the story of a game but would like to keep just the multiplayer mode, supposedly I'll be able to keep just that portion, which will save space.