I have another 8 "marksdwarves" who are absolutely useless; half of them won't even pick up their equipment, the other half won't fire their bow in any situation. Yes, I have done every single thing there is to get them to function. They have ample equipment to pick up, over two thousand bone bolts not stored in bins (some people say dwarves won't pick up bolts stored in bins, so I tried that), they have an armory with Individual and Squad Equip enabled and their future equipment is stored in it, yes they have quivers, yes they're set to train yes they have uniforms with crossbows and armor designated (not individual choice ranged) , none of it matters. Most don't have anything except clothes on, and even the ones that have equipped their gear refuse to fire it in all situations.
I also have a squad of 6 full legendary siege operators working four full masterwork ballistas aimed down my main hallway. While not technically military, they're only allowed to train siege operating and pump operating (for strength stats) and are more effective than the rest of my military combined.
All in all I have 22 dwarves assigned to full time defensive training. In a fort of 120, that feels fair. It isn't like it would have mattered if I had a thousand terminator dwarves or 8 in this fight, though; the goblins all hit the first dwarf to reach them, and since there is no option to hold ranks your dwarves charge in single file with your most agile dwarves taking all of the heat and the rest being ignored. Thus why my three best died -- they were the three fastest.