I explored the area around New Tristram, and I don't believe there are only 15, although this is a hard one to answer definitively.
- On Old Tristram Road, before you reach the ruins, I found 4 including one in a Damp Cellar.
- In the Old Ruins, I found another 4
- At Adria's Hut, I found 5 corpses, 2 inside and 3 just outside.
(The guy in the top right looks different, but I have another screenshot where I'm hovering over him and it says he's a militia member...)
This means there are 13 on the road to the Cathedral.
I also investigated Cathedral Level 1, and found:
- 2 fighting monsters near the entrance, alongside a villager. They died in the melee. They were wearing the same blue tunics and armor as the other militia members, although their corpses weren't lootable after they'd died.
- 1 uttering his last breath later in the area, who was wearing red, like the adventurers typically do. However, the closed captioning identified him as "Tristram Militia"
If we count these 3, the total is now 16.
Other Misc Notes:
- In the event under Adria's hut, Leah exclaims "Captain Dalyton!" before a group of undead attacks your party. Given the number of militia members in the area above the hut, and that the only people with rank in the area are militia, it's not out of the question to assume that the Captain was a militia member as well. Presumably at least some percentage of the other undead that attack are former militia members as well, although this is hard to quantify.
- I checked the path to the cemetery, but I found no members of the militia on this path. There were several dead villagers and a few dead adventurers, but no militia members.
(I've got more screenshots than this, but most are relatively boring.)