I have like a dozen sites. The hardest part is to determine, which ones will be those that you’ll work on the hardest (read those that will earn). If you have some very premium domains it’s easier, but if all of them are quite mediocre it’s confusing. I have a ton of domains like that and it just blows my mind.
On the other hand, large amount of sites helps for the obvious link building factor and makes you feel a little more safe. If anything goes wrong, you have a handful of sites that you can sell of vs. if you have 2 sites and they get a banana by google, it might turn out bad.
Hope that makes sense, just woke up after a 15 hour work day.