There is no specific limit that I am aware of. It's more about the content and your machine. Lots of images might not help.
Generate to your local drive if you are not already doing that.
350 should be OK and sometimes one topic can cause issues. Is it stalling at the same point? Have you tried using two layouts each with one half? At worst you may have to have two layouts and then combine them back in Word but trying that might identify a rogue topic if one layout still gives grief.
See www.grainge.org for RoboHelp and Authoring tips
Thanks, Peter. I had not thought of a new layout. I’ll give it a try. Mary