Average attendance during the latter Delle Alpi years was around 35,000, over a 10 year period it was about 40,000. The club miss out during the big games, especially in the CL, but the current capacity is enough generally. The atmosphere is created by having it full every week. That was the design plan, better to have it full than be 60,000 and have 1/3 empty.
I think as time goes on and with Juventus wanting to be a consistently major European player there will be even more demand from overseas and national fans for tickets, so they will have to look at expanding the stadium. As of yet they are not filling absolute capacity for games against Empoli and Frosinone, over the season the average is still around 38,000 - 39,000. I still think there is an issue in regards to less local fans going to the games, which is probably a spillover from the Delle Alpi days, but there is also a general culture in Italy of watching games on tv.
The other thing - tickets are quite expensive. Compare the Italian and German economies and then see that the absolute cheapest adult ticket for the Empoli game (curva) is €35, top end (halfway line) is €110:
http://www.juventus.com/en/tickets/.../31-juventus-vs-empoli/prices-table/index.php
Just looked at Bayern and the equivalent cheap ticket is €15 (there is even a restricted view one for €5) and the halfway line is €70. Don't you do a season ticket for €150? Last time I looked at Juve season tickets they started at over €400.