To add a little more color to my earlier answer of "One".
The AIA limits in certain cases the number of parties that may be joined. Here's one of many articles on it: http://www.finnegan.com/resources/articles/articlesdetail.aspx?news=2efb05db-3efd-498e-9622-6ad11c5c5828
"The Leahy-Smith America Invents Act substantially revised many aspects of U.S. patent law. In one provision, the AIA created a new joinder statute at 35 U.S.C. § 299. This provision was intended to "address[] problems occasioned by the joinder of defendants (sometimes numbering in the dozens) who have tenuous connections to the underlying disputes in patent infringement suits." H. Rep. No. 112-98, Part 1, at 54. To that end, the joinder statute makes it more difficult for a plaintiff to join multiple defendants in a single patent infringement action or to consolidate multiple patent infringement actions for trial. Id.; 35 U.S.C. § 299."
"The new joinder limitations in the joinder statute and the Federal Circuit's recent interpretation of Rule 20(a)(2) in In re EMC Corp. should enable defendants to successfully challenge joinder more easily in patent cases subject to the AIA. On the other hand, courts have recently found that the joinder statute "has resulted in a significant increase in the number of 'serially' filed patent cases" that burdens the courts' dockets. See, e.g., Consolidation Order at 1-2, ROY-G-BIV Corp. v. ABB, Ltd., No. 6:11-CV-622-LED (E.D. Tex. Oct. 1, 2012). For purposes of judicial economy and case management, courts appear to construe the joinder statute such that its restrictions on consolidated actions "for trial" do not preclude consolidation of pretrial issues under Rule 42(a), consistent with multidistrict litigations. Consequently, defendants in post-AIA cases accused of infringing the same patent or patents in a district court may find it easier to avoid joinder, but harder to avoid pretrial consolidation."
