Two plates mounted with 6 bolts. How to ensure 6 bolts withstand the force acting on the plates? and how many bolts required to carry the force acting the plates?
Calculate the force acting on the plate, then divide by 6 to find the force each bolt will sustain. Multiply by a safety factor of 3. For a grade 8 fastener, use 45 ksi as the allowable stress and calculate the cross-sectional area, then the resulting diameter. Use the next nominal size greater and you're onto determining the shear area and resulting length of the screw.
If you don't have a Machinery Handbook, you should get one. Much of this is covered there. I don't take issue with calculators, because they're quick and effective but you should strive to understand the math behind them first.
Garbage in = garbage out.
As cmalco has suggested, a good place to start is the Machinery's Handbook. This is a tool that any serious CAD designer should try to have. Purchased new it can be expensive. Good 2nd hand copies can be purchased cheaply if you look about for them. If you are likely to be involved with producing finished designs / drawings of structural steelwork having a reference library of specialist texts can make producing a good finished product of a design / drawing ready to go to a structural steel fabrication workshop to start manufacture. I have found the finished product is what customers like to give you money for.
From my library I have pulled a book " Structural Steelwork Calculations and Detailing ", i have attached image of cover and a few pages from the chapter on bolted connections. This chapter deals with calculating the various types of bolted connections, it being important to use connections the right way and in the right place. This book also helps with specifying and producing drawings that fabrication contractors are happy to work from to make the finished steelwork.