How many layers/samples of hi hats do you use?
It depends. Sometimes a track only needs one or two hats, sometimes you want a nice alternating rhythm between a few similar sounding hats, and sometimes you just want a burst of pure white noise!
How do you pan them?
It depends. Some tracks I have it set up like a traditional drumkit, where I pan them as if i'm sitting in the drummer's seat or I'm in the crowd looking at the drummer, some tracks I have them panned just for stereo spread, and sometimes I automate the panning or use an autopanning plugging to have it shuffle around the stereo field!
Do you vary the velocity?
It depends. Some tracks I want to have some more realistic hats or syncopation, and some tracks I just leave everything at 127.
What does the stereo width usually look like?
It depends!
Starting to get the idea?

I'd suggest listening to a lot of songs that you enjoy and copying how they do it for a little while, until you feel confident in how you could do it yourself!