If your genetically gifted with good chest development then one proper chest session a week will be sufficient but if you're not then twice should be the goal.
Depends on how well you're chest is developed then either flat and/or incline could be alternated between workouts. I personally need to do both because of my bad genetics with chest and when on cycle I aim for two sessions a week.
I train chest once a week, as my genetics are pretty good for the entire upper body. I like to do an alternation of flat bench and incline dumbells with incline bench and flat dumbells
That would be our pec major I think you should concentrate on flat bench with some other exercises included such as weighted dips,push ups, flys, chest press etc.
