I don't know about the export limit:
However, there is a bit of a glitch. Amazon will only store and show a
limited number of my “clippings” from the book on my Kindle page
(kindle.amazon.com). Publishers and authors are rightfully wary of
people clipping an outsized portion of the book and displaying it
but you can just copy
My Clippings.txt via USB cable. Also, you can share on twitter or facebook quite easily.
For Kindle books you should be able to e-mail notes to yourself.
thufir@dur:~$ ll /media/thufir/Kindle/documents/My\ Clippings.txt
-rw-r--r-- 1 thufir thufir 82597 Mar 26 15:31 /media/thufir/Kindle/documents/My Clippings.txt
thufir@dur:~$ tree /media/thufir/Kindle/documents/My\ Clippings.sdr/
├── My Clippings2c160740cd13972662f6cb2a18bbd068.mbp1
└── My Clippings2c160740cd13972662f6cb2a18bbd068.mbs
0 directories, 2 files