One bullet journal or many?
Initially, I started with a single bullet journal:
I followed the BuJo system religiously for almost a year, trying to put my entire life into a bullet journal. It stopped working for me. I found it too distracting to have my day job, my personal stuff, and my novels, all in the same Leuchtturm A5 hardcover notebook. After racing through notebooks alarmingly quickly — when I got to two notebooks a month, I gave up. In the time I was copying Collections to new notebooks, I could have written a new chapter.
If you’re just starting out with the bullet journal system, I recommend using a single journal, until you realize that you need more.