A few thoughts:
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.
Happy Friday!