You can’t do this in a simple way because pages are not primary objects in Writer. Pages are a consequence of text content. They are allocated on demand to host text. Delete text, there is no longer any need for pages.
To keep 200 pages, you must have content for 200 pages worth. At a minimum, you should add manual page breaks.
However, this is silly. Edit your question to explain why you want to keep a 200-page blank document. If your only motivation is to have a void skeleton file where you later add content to make it similar to another document in your collection, you should consider creating a custom template fully configured with styles. Styles may be used to control paragraph formatting but also page properties such as orientation, margins, header and footer. Images live in frames and there are frame styles to control their attributes.
A template may also have initial content repeated on every document created from it (skeleton cover page, copyright notice, skeleton TOC, chapter outline, skelton index, backcover, …). Initial content plus automatic page creation through paragraph style command should do what you expect.
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