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Without external links, Google will only find the site if you use the "Submit your site" option. At the page where you can enter the URL there is the following statement
We do not add all submitted URLs to our index, and we cannot make any predictions or guarantees about when or if they will appear.
My guess is that without any external links the number of indexed pages will be in the range from 0 to a few dozen. And even these pages will not stay in the index if no external links are found.