Cross-posting should be limited to a small number of directly related groups. You can cross-post your message to more than one newsgroup if your message is relevant to more than one group. Cross-posting is easy to do, and similar to sending email to more than one person. Simply put a comma between each newsgroup name you wish to post to in the address field.
However, remember that excessive cross-posting is considered bad form without a good reason, because it multiplies the traffic on the Internet several times without adding any new content. In the extreme case, if everybody cross-posted every message to every group, then every group would look exactly the same!
You should cross-post only when really needed, and usually not to more than three groups. All cross-posted groups should be closely related. It is considered very bad form to cross-post a message to unrelated groups, and could cause you to receive a large number of complaints by email.
Because cross-posting of commercial advertisements has become a problem, some news servers will ignore any message cross-posted to too many groups — depending on their configuration, somewhere between five and fifteen is the usual limit.
When you cross-post an article to more than one group, you may want to specify that all responses should be directed to a single group. If your software supports it, this is done with a “Followup-To” header that specifies the group that all replies should be directed to. If you set such a header, it is also considered polite to mention that fact in your message, with a statement something like the following:
(Followups redirected to alt.misc.thoughts)