Attic ventilation

My wife and I are preparing to replace the roof on our house. We live in central Texas. It is 2139 sq. ft. with a single space attic. We currently have two 17" attic turbines as our only ventilation. We plan to add soffitt vents but do not know how much we need. Also, are the two turbines enough or do we need to add ridge ventilation as well? If so, do we need a continuous vent the length of the house or will less do the job? The ridge is 71' counting the overhang. Thanks for any help. H