Each herf is different and I guage how many cigars to bring based on how many people will be there. I always bring at least one cigar to give to each "herfer". Then I bring cigars for trading and some for me to smoke. The last herf I was at I took about 40 cigars and there were 14 people. Unless someone seems like they want me to smoke a gifted cigar right away, I don't smoke gifted cigars at a herf. I like to wait until the mood strikes me for a particular cigar. That's just me. Stay at the herf as long as you like. There generally isn't a hard and fast rule about how long to stay. It's also a good idea to have a general plan on what cigars you are going to smoke. Let's face it, after a couple of cigars your taste buds are going to be on overload. So smoke any special cigar that you are saving for a special occasion first, that way you'll taste all of the flavors. Honestly, after the second cigar, I don't taste much anymore.
As far as sharing cigars while lit with others, it's not something I do. I have herfed with one guy that always offers his cigar for others to try. Most people say no and that's that, no big deal. To each his own I guess, but not something I do.
Kevin