I started a 2nd character with a melee build, and thought it was worthwhile to put together a list of all the ammo I've acquired to use as a guide for future characters. The character is level 25, and has used a fair number of fusion cells, and a few Molotov cocktails and frag grenades, but nothing else. No ammo has been bought or sold. I collect all ammo that I see, and I don't farm.
Every playthrough will have a fair amount of variability, but the relative percentages should be approximately the same each time. Some ammo only shows up after you reach a certain level threshold, obviously. This isn't a complete list, just a list of everything I've seen as of level 25.
EDIT: The numbers below don't include ammo looted from shotguns.
Fallout 4 Ammo Loot Count - Level 25
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.308 709
.38 4488
.44 222
.45 2246
.50 Cal 169
10mm 2074
5.56 381
5mm 17
Railway Spike 9
Shotgun Shell 556
2mm EM 0
Cryo Cell 13
Flamer Fuel 271 (893 additional fuel picked up from the Forged quest @ level 26)
Fusion Cell 2000 (est)
Gamma 114
Plasma Cartridge 144
Fusion Core 36
Mini Nuke 12
Missile 43
Cryo Grenade 4
Frag Grenade 46 (est)
Molotov Cocktail 64 (est)
Nuka Grenade 2
Plasma Grenade 2
Pulse Grenade 5
Bottlecap Mine 6
Cryo Mine 8
Frag Mine 35
Plasma Mine 3
Pulse Mine 9