No specific navigation structure. Although its ideal to have 20+ pages for a beginner level website. No site structure either just keep the site enriched with keywords and have a minimum of 300 - 500 words per page. I hope this helps.
Yes, there is no specific site structure to be followed in SEO. Your site should be structured in the way that it should friendly to both search engines and users.
Don't forget to optimize the web design and the images, as they're part of the general structure too. If you have doubts, use a template until you get a general idea of how a website should be structured. Most of the serious websites out there are a fairly good example.
It depends upon your intelligence and sense of beauty. You can do seo even for one page. The structure does not matter.
A site structure I believe is basis of SEO. You need to identify the primary keyword for your site. That keyword should be included in your domain name. This step is very critical for SEO. Also the categories and pages on your site should be structured around that primary keyword. So if you building a site for famous tourist destination like Isle of Wight, then that will be your primary keyword. The categories will be like hotels, maps, transport, tourist attractions and so on. Use Google Wonder Wheel tool (which is free) to identify the possible site structure. Read the latest article on searchtechword.com
I can add that if you have multiple websites, you can simplify the process of creating and submitting Sitemaps by creating one or more Sitemaps that includes URLs for all your verified sites, and saving the Sitemap(s) to a single location. It can be worth for SEO.