This question is about the last episode of season 5 of Orange is the New Black, read at own risk of spoilers.
At the end of season 5, after the riot team has cleared the prison, they report that they are missing 10 people. Caputo informs them that they are probably in the swimming pool.
The people who are hiding at the swimming pool are:
This makes 10 people...
But Tiffany 'Pennsatucky' Doggett has left the prison prior to the riot team's assault and is in the guard houses with Charlie 'Donuts' Coates.
This makes 11 people...
But, Mei Chang has also escaped the prison prior to the riot team's assault.
This makes 12 people...
But, Linda Ferguson, Employee of MCC is arrested and counted as one of the inmates, so they are counting one person to many.
But this still leaves us with 11 people missing, not 10.