How many vectors does paragraph vector generate for each paragraph?
It would make no sense of the word embeddings to change within a document. It would be as if the spellings of the words changed; how would that help? 

When you use a document embedding, you find its numerical representation in a way that loosely captures its meaning. If you want to capture the meaning of each paragraph separately, find their embeddings separately. If you want to capture meaning of the entire document, feed the entire document.

For example, if your document covers a range of topics and you want to allow users to pinpoint where a particular topic is covered, you can find embeddings of each section (paragraph, page, etc.), then find the section that's nearest to your query's embedding.

One use case for dynamic word embeddings is to identify temporal dynamics; how the meaning of a word changes over time, as in this paper: The Visualization of Change in Word Meaning over Time using Temporal Word Embeddings.