Is there any way this count(512) could be increased?
No. This will have been set when IE's code was written. The IE developers will have chosen 512 characters as a sensible maximum for the title
attribute.
The fact that you need to store more than 512 characters in it is a code smell and you should be looking at storing that data in a different way, or you need to shorten the string yourself to keep only the data you need.
Stackoverflow, for example, only shows some of a post when hovering over it on the homepage, not the whole post. The tooltip would be too large for one, and some browsers hide the tooltip after a short amount of time making reading up to 512 characters let alone more very difficult.