Number of Bolts vs Size of Bolt

It shouldn't be necessary to calculate the number and size of bolts in any pipe system. The information can be looked up in tables.

Suppliers for piping materials will provide standardised flanges in accordance with the tabulated schedule. Scheduled flanges with scheduled bolting with scheduled joints and the bolts flogged up to the scheduled torque should provide 100% sealing at the scheduled pressure.

To do otherwise is like reinventing the proverbial wheel.

let me elaborate,

I am not redesigning the customer side flange. The question is for bonnet side flange of valve.

#1 still applies.

"...bolts flogged up to the scheduled torque should provide 100% sealing..."

Nope, nada, nein, nyet, non. Unless, however, the stars are aligned, the teas leaves float in a certain pattern and you are very, very lucky.

What you need to keep in mind is that even if all the specified fasteners on a specified flange compressing a specified gasket are tightened to the specified torque, each and every one of those fasteners may be completely loose. Okay: that's a bit of a stretch (although not impossible). What's more likely is that some of the studs might be too loose, some might be too tight and some may be just right - even though the same torque was applied .

It's not applied "torque" that is the important thing here. Rather, it's the bolt clamp force. The two are NOT the same.

Greater number of smaller bolts to acheive the same degree of gasket stress whenever possible.