I also think 13 is a good number because that would be 1 Supreme Court justice for each circuit court
But getting to that will be hard and not to mention unless a cap is put in place (I prefer tying it to the number of circuit courts) then the next person who scoots in could expand it further with less push back due to it having been done just recently
The last thing we need is every president who scoots into office appointing more and more justices until it gets out of hand