I suspect no one yet knows for the pig and it may vary between individuals and breeds in humans and mice. Given Th17 cells also play a role of sentinels in the gut, infection status with gastrointestinal pathogens may also influence this.

Pigs, I'm not sure. For uninfected, SPF mice it depends on the vendor (See Ivanov 2009, Cell). Humans , around 6% (See Farber, 2013, Immunity).

The frequency of Th17 cells in the gut largely depends on the type of gut bacteria in human as well as in animals (Yang et al, Nature letters 2014; Goto et al, Immunity, 2014).  Same strain of mice from different vendors may have different gut bacterial composition, therefore different frequency of th17 cells.