1) if you don't know, you should be working under someone who does. If that person asked you to look into this, s/he probably had more research in mind than an internet forum.
2) The most direct reply is: however many it takes to resist the forces in the connection, whether axial, shear, or moment.
3) On the chance that you're asking about shear connections, there are OSHA concerns (erection), and there are geometric concerns (beam end stability). I recommend the AISC Steel Manual, Part 10: Design of Simple Shear Connections as an excellent primer of how to make it happen.
A few popular rules of thumb (after you have determined how many are required to resist the load):
1. Minimum of two bolts per connection
2. However many bolts you can fit in the beam web spaced at 3" apart over the full beam depth
3. Same as 2 above, but if you have restrictions that prevent bolts from being installed over the full beam depth, always try for at least bolts over half of the beam depth.