When I was in the military, you couldn't have more than 25% of visible skin tattooed, and a waiver was required if you had any above the collar. Now it seems like they have lowered the bar a bit. There are numerous people on the youtubes that have fun little stories about their time in combat with 2 full sleeves and a neck piece that could never be hidden by anything but NBC gear. I'm sure some got their ink after the fact but it seems pretty common among modern vets.