Q:

What process can generate a new write sequence

While compiling a very large document recently in xelatex and tex4ht I have observed that tex4ht doesn't handle write streams the same as xelatex.  I am trying to prepare a MWE to troubleshoot my problem that started when I added another type of glossary to my document.  But I am unable to reproduce the problem in smaller documents.  
I don't think it is is specifically a glossaries problem because the addition of this sixth glossary doesn't break in a MWE with only 1 example term of each.  My broken project  has nearly 10 thousand entries in a variety of files loaded before the \begin{document}.  I am unaware of any entry limit in glossaries that would force a cap on the number of entries that can be searched within but I suppose this could be possible and difficult to isolate unless known about in advance.
I make pretty full use of TeX automations as a whole:

cross references to document hierarchy using \ref{}
cross references to figures, tables, and equations using \cref{}
bibliography management for distinct parts, chapters, and sections using bibunits
table of contents and lists for all other types of objects
Six distinct types of glossaries, and the corresponding \makeindex, and \makeglossaries and printglossary[] calls for appearance customization
I am sure there are others I use that I am missing.

I have tried to force write calls by including at least one of each in a MWE but I am unable to crash my MWE.  
Could anyone provide any advice regarding:

what other types of package calls create a new \write? 
Or what causes distinct new \write calls to be made multiple times from the same package.
Or point me to documentation that describes tex4ht limitations regarding new \write? 
Maybe all this is unnecessary if tex4ht can be forced to permit extra new \write streams...

A:

A while ago I ran into this issue in a different context. Searching for answers, Stephen's suggestion is the one which I ended up using. morewrites enables more new writes.

