Answer:
This is based on my years of translating documents to and from English. An average sentence is a vague concept and depends largely upon the level of education of the writer, context, style, and purpose. Scientific writing is typically containing more words in each sentence.
So, in a typical sentence, there is one or two nouns at least, a couple of verbs and adjectives, and a handful of them two-three-letter-long articles and prepositions. We’re looking at 5–7 words per sentence, give or take.
This pseudo scientific approximation gives us a hundred sentences in a 500-word text.
Good luck with that essay!
