I was wandering, what gives more server load - many categories or too many items in one category. I.E. I have a list of magazines, there are arouns 100 names, each one publishes 12 issues per year, and I plan to cover at least 20 years of publications. What would be the best way to publish - one category and then regulate by extra fieds, or separate by magazine name? What will give less load on a server?

Actually more traffic will give you more server load.

I would go with several categories (or even tags) which is more easy to manage, and then I would enable Joomla!'s cache. If your server setup is properly configured you won't have an issue.