Dwarfs will claim one non-armour item for every layer on every body part, if at all possible:
Body parts are upper body, lower body, head, hands and feet. Layers are under, over and cover.
For body parts that come in pairs (hands and feet), one individual item for every limb present is claimed.
Dwarfs will not by themselves claim or wear more than one item of a layer, no matter what size permits say - a civilian will (in .34.11) not claim and wear six cloaks.
There are some restrictions:
- all "under" clothings for lower body and head are "exotic"; they cannot be produced by the dwarfs themselves. If any such stuff comes into a fort, the beards will claim and wear it, but cannot create replacements, which may lead to a moderate amount of unhappiness in the proud wearers of XXtroll fur loinclothXX.
- some bodyparts only have two layers of clothing available - lower body and feet lack a "cover" layer (and, see above, the "under" layer on the lower body has no clothes made by dwarven civs), for the hands, there's no "over" layer.
At embark and immigration, every dwarf has a clothing item for every body part, for every layer the civ can provide for. As soon as any clothing item starts to show wear ("xcave spider silk cloakx" etc.), a dwarf starts looking for less-worn replacements. Civilians will only discard worn-out clothes after claiming a replacement, otherwise they typically wear items until they disintegrate, complaining all the way. This can be quite annoying in the case of clothes you cannot produce, like loincloths.
There's an amount of variety in upper body clothing - all civs know how to make cloaks, the only "cover" for the upper body dwarfs can produce, but the types of "under" and "over" clothes available varies from one civilisation to another; at least one kind of clothing for each layer appears guaranteed. Immigrants and embarkers are still restricted to clothes the civ can make, as far as i'm aware.