I've found the mission rates to be the days between sorties.
As regards the max missions figure, if memory serves me correctly, it is the maximum number of missions before the campaign automatically finishes based on 'normal' campaign length setting. Altering the campaign setting to long or short will alter this figure, while achieving mission objectives consistently (and thus capturing your goal(s) in enemy territory will shorten the campaign).
As far as activity during ground attacks are concerned, I would recommend examining Charles' euro1962 campaign as I've had several hunter CAS sorties getting tangled up with su-7 CAS's and escorting migs. I suspect it's a combination of overall supply values being large (enabling continuous ground war until your ground units run out!) and setting of AI squadron mission types.