It could be N different licensing companies with the patent holder paying commissions. It could be scammers having no relationship with the patent holder at all. It could be a mix.
In a court action, the plaintiff must assert that they are either the patent holder or are holding some actionable interest in the patent (such as a license that includes the right to enforce the patent). The defendant can contest that assertion and thereby require the plaintiff to provide proof. Normally, no subpoena is needed (subpoenas generally go to third parties, not the plaintiffs/defendants). At worst, just a normal discovery request is needed. If the plaintiff outright lied about this then the judge is likely to get medieval. Only a very foolish law firm would fail to check the patent's chain of title and their client's right to sue.
Similarly, the plaintiff must clearly specify the defendant's infringing action. Failure to do so can result in near immediate dismissal of the case plus sanctions. This is particularly true with patent enforcement. That is one reason the Project Paperless letter is so laughable. It tells the defendant to do the investigation and then send money. If they sue because, as the letter says, "your company fits the profile" then the law firm filing the action could well get sanctioned without the defendant ever formally answering the complaint.