Is there a limit to the number of entries/items in an RSS feed? Should I have 10 entries only? Or can I go up to 100 for example? What if I have more entries than 100 per day? What can I do?
Depends on version of RSS used. If you're using the UserLand RSS 0.91 spec, for example, the number of item in a channel should be limited to 15 according to info in the RSS 2.0 spec. If you think of RSS as a format for periodic updates this makes sense—though it can be limiting.
If you look at jekyll-feed RubyGem—which uses Atom and is deployed on GitHub Pages sites—the number of posts is limited to 10. But you can do whatever your want as long as the spec permits.
For example, if you have more than 100 entries per day you're obviously going to want to increase the number and RSS 2.0 (and maybe Atom?) is fine with that. To signify to the RSS user agent the content is updated with a high frequency you can use the Syndication module to output time:
<sy:updatePeriod>hourly</sy:updatePeriod>
<sy:updateFrequency>1</sy:updateFrequency>
(Don't forget to add the XML namespace when using.)
Just keep in mind if you're using the Content module to output the full text of an an article in a CDATASection you're probably going to want to truncate some text if the sections are large.
Can I have pages with each page displaying 10?
Yes, though if you do you should probably have an RSS for your RSS to indicate the pages and use guid without a URL to ensure they're properly identified and deduped by the feed reader. This is also going to depend on the RSS user agent and what your desired results are.
And just for fun here are some modules for RSS 2.0 which make it very extensible:
xmlns:content="http://purl.org/rss/1.0/modules/content/"
xmlns:wfw="http://wellformedweb.org/CommentAPI/"
xmlns:dc="http://purl.org/dc/elements/1.1/"
xmlns:atom="http://www.w3.org/2005/Atom"
xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
Only use what you need and drop namespaces you aren't using.
