I have one main site selling my services. I plan to register another 10 sites on different C Class IPs and link to my main site. Please note that all 11 sites (main + 10) have different content and are well written with relevant and distinct information.
Does Google consider this as spam or will it help rank main site better?
AFAIK a website's IP address is not by itself a ranking factor. What search engines can do is identify 'bad neighborhoods' of low quality sites that link to one another. Your 11-site linkfarm might be identified as such.
Also, all those new domains will have no authority of their own, so links coming from them won't be helping your main site much.
Yes Google does consider it spam. Not sure if they do anything about it because it is not very good spam. It would be much easier to just get ten links to your site. It is never a good idea to create more than one website unless you can get links on another that site you can't get on yours. Those links would be worthless until you built up links for each of those sites. Just focus on your main site. Many people try this doing splogs and other methods and the ROI is just very low. Spend some time learning how to get more links. I used to be a big spammer and white hat is a lot easier. I don't have to worry about sites getting banned any more. All the work I put into a site stays and my rankings keep getting better.
It's perfectly OK to link to your own resources within reason. If additional 10 websites have a value of their own and the link is relevant. But if these additional 10 websites are junk, sooner or later it may come back to bite you.