Every serie A team can sign during the season (it includes the summer and the winter transfer session) only 2 non-EU players from OUTSIDE serie A.
For example, Juve can sign 11 players with non-EU passports, but in that case 9 of them will have to be bought from other serie A clubs.
It's interesting that in the last 2 seasons we never used our right to buy a non-EU player outside Italy.
In 2007/08 we bought Salihamidzic (Bosnian but with German passport), Sissoko (French passport) and Almiron (Argie but bought from another serie A team so doesn't count as non-EU)
In 2008/09 we bought Amauri and Knezevic (from another serie A team) so again we didn't use the non-EU players quota.
Of course, you can buy more than 2 non-EU from outside Italy, but you can register only 2 of them for the particular season.
There is also a loophole. You can sign a player, loan him to another serie A team, and then get him back as a non-EU because you're bringing him from another Italian team. (we did that with Mutu in 2004)
Milan on the other hand were screwed hard for not being careful with the non-EU rule. They registered Shevchenko and Vuidez and then they didn't have space for Thiago Silva so the Brazilian couldn't play at all for Milan, even though he was much more needed than Sheva and Vuidez.