Currently, the minimum bounty is .001 ETH for each recipient.
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The number of recipients is simply the number of users who receive a portion of the bounty for providing the best replies as chosen by the community.  The number of recipients is chosen by the creator of the post before publishing it. 

For example, if you want 20 users to be rewarded for responding to your post, you'll need to attach a bounty of at least .02 ETH.