I did in case my kids wanted to show their kids one day. My brother did because his 2 kids will fight haha. If you might split up because the kids want autographs from different characters, then you'll want one for each.
Now that the kids are older we just do one. When they were little they each had once. We once found our youngest sitting at the table in our room scribbling in his autograph "like the characters do". That really is a special book now.