I don't think there is a limit, but I haven't really stressed it. I have definitely had ten or so annotations.. I did just try a test for a book that has 22 screens of annotations. But, that showed a bug in the feature.
The bug is because I have two copies of the book on the device. One is the kepub version and the other the epub. The annotations are on the epub, but as I have finished the book, I marked the kepub as finished. When I used the fetch annotations, it got the annotations for only one of the copies. Unfortunately, it was the kepub version.
I'll look at the bug later and have a look to see if there is a limit to the annotations as well.