Actually, I think 2-2 1/2 litters a year is the general rule, and feral hogs don't normally have as large of a litter as domestic. All things said though, they can be baby machines. The gestation period for hogs is in the general vicinity of 114 days. An easy way to remember this is what they call the rule of three.....three months, three weeks, and three days...or 114 days total.
Trying to keep up with all the sows out there though, is actually a losing proposition.