Yes, it comes with a 20-round Pmag and the only difference between that and other Pmags/normal AR mags is the length and capacity, fit is the same.

John, yes Sir it will. Most manufacturers including Kel-tec make firearm packages that have lower capacity magazines so their firearms can also be sold in state that have magazine capacity restrictions. The firearms are exactly the same but just have lower capacity mags included. The firearms will also accept any standard capacity magazines just fine.