Del Piero started to play in Juve in 1993 so he didn't start with #10 cause in seasons 1993/94 and 1994/95 # 10 was Baggio.
I don't remember which one was his number and also I didn't find it on the net. If you are really courios to know it, maybe you can look for his old pictures or goal videos, there will be one of his first seasons, I'm sure
That's why I tought that 9 is impossible... cause usually was Vialli number.
But maybe he had a "proper number" like 4 Baggio was 10, Vialli 9, Ravanelli 11, ecc... Anycase it's very difficolt to find out which one it was the first he weared.
The only way it will be to figure out wich was his first match and to find a picture or a video about this match...