There are no such restrictions. Theoretically, you can sync hundreds of calendars, so I guess there’s another reason why some calendars don’t show up. Do those calendars have something in common, for instance are they shared calendars?
Are you sure this problem is related to DAVdroid? DAVdroid just shows the events from subscribed calendars, regardless of whether the calendar is shared or not. So, as soon as the server sends the events for the shared calendar, they should show up in DAVdroid. If they do not and there is no error message, it’s probably this server problem.
I just found that while the shared calendars can’t be viewed and thus not edited…Where read/write access to the shared calendar applies, calendar events can be created in the shared calendar on the phone and are shown in the Zimbra web based calendar after sync completes.