How many days should be my transit visa in the application, and what's the purpose of choosing HK?

I live in China, and I will book flight tickets to HK then from HK to my final destination. So, basically these arrangements are made by me not the airlines as connection or stop over. I'm planning to apply for transit visa but I was stuck in the purpose of choosing HK as transit place, and do I need to mention the day I go back to HK and then back to China? or just once is enough with two entries?