I apply for an F-visa (short term internship), which intended numer of entries should I select?

I have an invitation letter for 82 days, can I apply for a double-entry one for 3-6 months and will this make it possible to stay 82 days? Or will I be able to stay only 30 days after entry and therefore would need to apply for multiple entry one?

Also: In the application form: which box should I tick for an F type at "major purpose of visit"? (Official Visit/ Non-business visit/ Other (Please specify) / Short-term study for less than 180 days (I think this last one is for X1/2 type)?