I doubt you will ever have a problem (even if heavily linked as long as the content (e.g. "multiple sites" infers "different sites") and not just the same stuff under a different domain - with slight alternations.
It does not matter what the content of the site is honestly. If they are the exact same that is obviously a problem, but if they are slightly different thats ok.
When a site gets in trouble for cross linking is when it has tons of cross links
AND few external links point into the network.