There’s a difference in how many I run and how many I own. 
I own about 10 currently, but run 4 more for others (my church, a local car show, etc). Of the ones I own, one requires almost zero upkeep, 2 others require very minimal upkeep, a couple of hours a month to update a datafeed on one and add new content on another.
4 of the others are fairly new (within a couple or so months) so I still work on them here and there. And the rest, including my oldest site, don’t really need work unless I just choose to do so.
I typically don’t mix my sites with each other since they’re all of different content, but most all of them do link back to my oldest site. I do link all of my sites from my oldest site, but it’s mostly just because Google visits it often and I know they’ll get crawled and indexed quickly (I still submit to Google too, but it really isn’t necessary).