How to set number of defect opportunities ?

There are basically three ways: DPU (Defect per Unit)
DPO (Defects per Opportunity) and Defectives (Pass or Fail). Which ever method you choose is correct. In most cases, it depends on the current data collection methods. However, DPU will probably provide the most information.

Here are my thoughts on classifying the opportunity and defect:
>1. on typing a letter.
opportunity is the letter or word or page, your choice
defect is a misspelled word or mistyped letter

>2. on assembling of a part with six bolts
opportunity is putting bolt in correct hold
defect is any bolt applied to wrong hole, or mis-tightened, or any other non conformance to customer specification

>3. on assembling an engine with 360 components
see above #2

>4. on an NC machine which has 12 item proceses and 200 units part processed a day
in simplest terms:
opportunity is a part
defect is any non conformance to the part specifications, regardless of how many processes are applied to each part