We have about 80 domains spread between 2 companies.
Although what the Professor says makes sense – we have been lucky that we have not been penalized by the major search engines but this may be because we generally only do minimal link exchanges with any sites purely because we don’t have the time. We don’t cross link sites within our own network…or should I say – we don’t do this anymore. We used to in the past but have removed the cross linking over the years not because we were penalized or anything but just because most people said it was not a good idea to do it. We do occassionally cross link across different IP’s and different companies .i.e we may put a link from one of our websites on our german server to one of our websites on our South African or USA server. We prefer having numerous websites as most of them specialize in a specific field – eg we have http://www.marvelslots.netwhich is only marvel slot games and http://www.mobileonlineslots.com which is specifically for mobile slots. I won’t do anymore in case it looks like I am trying to do some free advertising 
but suffice to say – this is one of the reasons we have so many different sites. We also have a team of web designers who get bored maintaining the same websites – so we keep them interested by giving them new websites to design.