Most of the celebs sign all 4 days but there are a few that will only sign a couple or will not sign one or two. Also they will take time out of their signing times to go to panels.
I will note that in the past if you only had a single day ticket you got into the hall later than if you had a weekend pass. Meaning if you wanted one of the big name celebs the line for them would be huge or even full by the time you got in.
I don't remember the previous Celebrations tickets selling out at least not that I am aware of.