It's 10, now 1000 at the time of writing this.
You may get a handful as you work through Leviathan corruption(I had exactly 1 after clearing corruption 10). However if you just run endless and manage to survive the first night, you should get both the 10 jellyfish as well as the "I Can Do This All Day" achievement for surviving the night.
If you need help surviving the night, please refer to my guide on that achievement.