For normal sem is 20 credit hours max. Im pretty sure it is the same for Special Sem.

Depends on the subject offered. I doubt you will be taking more than 10 credit hours for Special Sem because there are less subjects offered during that period(the ones that you interested to take).

The duration for special sem is only 7 weeks. So taking 6 credit hours or 9 the most would be sufficient if you want to score well.