I have a lot of websites on a server. Is it bad to link them all together? What is the maximum number of sites to have a on server before google gets ya and takes away some of the value or all of the value of your links! 5? 10? 20?
It is horrible!! Google's guidelines specifically states to avoid web rings. It is viewed as another form of manipulation. That said you can likely sneak by with only linking a few together, but why take the chance when there are such a large number of other ways to obtain links without breaking the rules?
If you are linking from one of your sites to another then do not use site wide links, next if you do want to link then write an article on one site and put a link in the article to another related resource on another of your websites, as an alternative create a free wordpress blog and use this as a linking mechanism. Yes it is against the rules but it is done more than most will state/own up to.