We will have lunch after hike on Tuesday about 1:00 p.m. Our group is about 12 people. Do we need reservation? (but we don't know exact time) Is there enough space for 12 cars?

Skyline does not accept reservations, but as a courtesy, a call-ahead is appreciated. There is plenty of overflow parking across the street.

You should give them a call directly to ask. Parking lot is pretty small, I doubt there is room for 12 cars. There might be enough space for 12 people people to sit together outside, if there aren't any other patrons.