Normally around 500 - 700. Just kidding really it all depends on the situation. As long as people agree to the trade it could be 5, 10 or 100. But normally I would think a few is enough 10 or 20 is plenty and maybe even overkill. You just want to make sure you get germination and a few more than you need is good for that. If you grow out the plant than you have a ton more seed when harvested.
If you only have a few to spare make sure it is known. Most people won't mind. By the way I have never had germination problems from any of the seeds I have got in trade. A few may have come out as something else but that is risk in saving seeds. Also you may want to note if your seeds have been bagged or were open with a chance of cross pollination.