According to the official PSL website:
If the five foreign players per team is true, that would account for 25 foreign-born players. However, there are already 24 players that have already been confirmed to be a part of the PSL. The actual number should be 25 as Michael Carberry has also announced that he will be a part of the PSL but his name is not on the confirmed list. I believe that Dwayne Smith has also been confirmed too, so that would bring the number to 26. Plus, Sangakkara has also expressed an interest in being a part, so I think there will be more than 25 foreigners playing in the PSL.
International signings in the PSL so far:
Australia: Brad Hodge
England: Kevin Pietersen, Ravi Bopara, Luke Wright, Tim Bresnan, Jade Dernbach, Chris Jordan
West Indies: Chris Gayle, Kieron Pollard, Darren Sammy, Sunil Narine, Dwayne Bravo, Andre Russell, Lendl Simmons, Denesh Ramdin, Jason Holder
New Zealand: Jesse Ryder, Grant Elliott, James Franklin
Bangladesh: Shakib Al Hasan
Sri Lanka: Angelo Mathews, Lasith Malinga, Thisara Perera
South Africa: Johan Botha
psl.cricket.com.pk/teams/