If you go to FAQ located under your name on the top right side of your account page, and many other pages, you can get the answers to any questions you have regarding what is allowed and what is not.
Here is what FAQ has to say (number 11) about being overly promotional in one's hubs:
11. My Hub was moderated for being overly promotional; what does that mean?
Purely promotional offers and Hubs designed only to promote other sites or businesses are not allowed. In particular, the following actions are likely to get your Hubs identified as overly promotional:
including more than 2 links to any one domain
linking to the same domain in the body of the text AND in an RSS feed
short "teasers" with links to "read more" at another site
linking to sites/pages/Hubs that are unrelated to your Hub's topic
including links to a page that contains largely the same content as your Hub
Please note that promotional links are links that you have any interest in promoting (your blog, your Website, affiliate offers, etc.).
Links to well-known Web resources which you don't have any personal interest in (like Wikipedia, news sites, encyclopedias, open directories, etc.) are exempt from this limitation.