Topics & Links Limits - Help Authoring Site
The limit is down to the help compiler but you can produce quite large help files e.g. > 5 MB.
If your document is very large then Word *will* run out of memory. It is difficult to say how many topics or links is the limit in this case but 300 topics and 800 links in a document should be possible but not recommended.
However having so many topics and links in a *single* document will slow things down and is not really a good way of doing it. You can make help files from multiple documents and even link between different help files so if you organise you files there really is now limit.