You can have 2 links per domain. Any more than that, and your hub will be labeled overly promotional. Other than that, I don't think there are any limits (I haven't encountered any).
Some things are exempt from that limit. For example, I don't think there's a limit on the number of links you can include to your own HubPages subdomain (i.e. your other hubs), and other "reference" sites (like Wikipedia) are supposed to be exempt.