Seven, so far.
1) Nicolae Todorache and Spiridom Ciorebea, residents of Glod, Romania, the village used to depict Kazakhstan, sued the filmmakers for $30 million claiming they were misled into thinking they were participating in a documentary about the village's poverty. Todorache and Ciorebea filed the lawsuit on behalf of their entire village. Todorache is the one-armed man who had a sex toy attached to his stump. Todorache and Ciorebea later voluntarily dismissed their case.
2) Christopher Rotunda and Justin Seay, two of the University of South Carolina fraternity brothers shown traveling in an RV, filed a lawsuit claiming "mental anguish and emotional and physical distress". The two men also claimed they were encouraged to drink alcohol by the production crew in order to engage in "behavior that they otherwise would not have engaged in.".A judge has since dismissed the case.
3) A South Carolina man filed a lawsuit accusing Sacha Baron Cohen of invading his privacy by staring at him in the bathroom of a restaurant. Footage from that incident was not used in the final cut of the film but was shown on the internet and the film's preview special which aired on Comedy Central.
4) Jeffrey Lemerond of New York City is the man shown running away in fear when Borat tries to hug him. Lemerond sued the filmmakers claiming "public ridicule, degradation and humiliation" and that his image was used without his permission. This suit has since been dismissed, Sources: http://imdb.com/title/tt0443453/news, http://www.reuters.com/article/entertainmentNews/idUSN0244908420080402
5) Felix Cedeno of New York City, who was one of the subway passengers, sued the filmmakers for $2.25 million claiming invasion of privacy and that his image was used without his permission. Despite this, Cedeno told the New York Post that being recognized by strangers is "pretty cool". Cedeno later dropped his suit.
6) Driving instructor Michael Psenicska filed a suit against Sacha Baron Cohen, One America Productions and 20th Century Fox claiming he was duped into participating in the film, seeking $400,000 in actual damages and additional punitive damages for misleading him and for emotional harm he continues to suffer. Source: http://www.reuters.com/article/entertainmentNews/idUSN0437222820071204. A New York judge tossed out this lawsuit claiming he accepted money and signed an agreement releasing the filmmakers from liability. Source: http://news.bbc.co.uk/2/hi/entertainment/7607752.stm
7) Etiquette coaches Kathie Martin and Cindy Streit, (the one Borat handed a bag of human feces to at dinner ) sued Cohen and Fox for allegedly engaging in fraudulent tactics to get them to appear on film. The judge dismissed both of their cases.
Gypsy singer Esma Redzepova announced that she was planning on suing the filmmakers for 800,000 Euros for allegedly using her song "Chaje Sukarije" without her permission. It is currently unknown if the lawsuit was filed or if the parties have settled.
British tabloid The Sun reported that Mariam and Joseph Behar, the couple that owned the bed & breakfast, were filing a lawsuit against filmmakers claiming that their business had been negatively affected. However, in numerous interviews the Behars stated that they enjoyed the film and considered Cohen to be a genius. The Sun later removed the story from their website.