Reducing Number of Bolts Results in Flange Thickness Reduction
Appendix 2 flanges consider two bolt areas: Ab is the actual bolt cross-sectional area; Am is the required total bolt cross-sectional area. The code requires Ab to be greater than or equal to Am, but it also penalizes excessive bolting.
The average of Ab and Am is used to determine the bolt load for gasket seating (W). As the actual bolt area increases, the value of W increases; therefore, the bending moment on the flange increases and drives up the flange thickness.
Decreasing your bolt area, Ab, to as close to Am as is practical will help you get a thinner required flange thickness.
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