You normally use 12 mm, unless the rim requires long ones, for example Mavic CXP 33 and CXP 30 do require 14 or even 16 mm nipples, some Chinese carbon rims do take 14 mm. For other rims, If you don't find the exact spoke length, you can compensate by going one mm shorter and using a 14 mm nipples instead of a 12
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have a pair of Disc wheels (MTB) i built with half and half 12 and 14mm nipples as i was short of the correct length spokes.
Parktools :?:SheldonBrown