First, To answer your question about how many.... two. More won't hurt, but one may not be quite enough.
When you apply a mechanical fastener, you are compressing the handle material in that spot. This causes the handle to try and lift slightly on both sides of the fastener. If you put only one in the center, it would have a tendency to lift at the ends. This may never happen if the epoxy holds and the fastener wasn't over-tightened ( they should be barely hand tight, never torqued down hard), and pins are used....but if things happen later on, like moisture warping and aging of the glue joint, it can lift a bit.
I like to use two Corby bolts on the ends and a mosaic rivet in the center on hunting knives.
On kitchen blades, three Corby bolts is a good choice. A neat look is the center being larger than the end two.
Now, why Corby style bolts ????
Because they come in brass, nickel, stainless, aluminum, bronze...and in sizes from 1/8" to 3/8"....in several lengths.
They are super simple to install and the finished look is flawless. All you need is a counterbore to match the bolt size. I suggest buying the counterbore from the supplier of the bolts, as there can be very minute differences in manufacturing.
Fast assembly with no clamps needed. This make multiple knife assembly much easier.
You can test fit the handle as many times as you want before final glue up.
The handle can be sanded to a perfect fit along the tang. This allows the tang to be fileworked after the handle scales are shaped to exact fit. On damascus blades, the tang can then be etched.
They make finishing the front of the scales easier. You just put the bolts in place without the blade and sand and polish the ends before glue up. A perfect match every time!
On handles with colored liners, you don't have to deal with "slip" when clamping them up at the same time. Ask anyone who has had this happen how many bad words he said while tearing the knife apart to re-do the handle.
They can be engraved to really make a custom pop.
Notes for the new user of Corby bolts -
Only tighten to just snug. There is no gain to making then super tight, and this can lead to handle splitting later on.
Test fit everything before glue up. If it needs adjusting to fit flush, do it and check the results. Once all is flush, glue up the handle.
Just like any handle assembly, a slight concavity of the scales makes a better fit. This way you are only dealing with the seal around the edges, and can get a much better fit. Just sand/grind/burr out the center within about 1/8" of the sides, making the center a few hundredths of an inch shallower. Just a little is all you need. Around the bolt holes, leave a 1/8" shoulder,too.
Don't put glue on the threads or in the female bolt. Just use a Q-tip or bamboo skewer to apply a little epoxy to the handle holes. Slip in the bolts and start the threads, Apply a little bead of epoxy to the head part of the shanks, and as you screw the bolts down it will smear the glue to a perfect seal.
Stainless is the hardest to finish, as overheating and burning of the wood can quickly occur. Go slow, file flush first, use fresh belts and slow speed, hand finish.
I have had pins lift, rivets pull free, screws come out, and glue fail...but I have never had a Corby come back broken. The handle, yes, the bolt, no.
