Do you need a word count for non-fiction query letters?
Sadly the question is so short that I'm not sure whether you mean 'Do I need to include a word count for my manuscript in the query letter?' or 'Should the query letter itself conform to a word count?' So my excitement at your brevity has now paled in the halogen glare of my confusion.
Just in case it's behind-door-number-two: ideally all query letters are about four or five paragraphs long - or about a normal A4 page in 1.5 line spacing for the obsessives amongst you.
If it's about the word count for the potentially unwritten non-fiction manuscript: yes, give an estimate, lest the publisher thinks they may be contracting something that's either 400 000 words long or 15 000 words short. You won't necessarily be held to the estimate, but contracts tend to contain a word limit so there needs to be something.