It really depends on how you intend to use the software. Scrapebox is so versatile that any given users usage scenario is often completely different from others despite that the basic overall functionality remains the same. For example, I don't use scrapebox for intensive keywords scraping, but for other purposes, and I find that the proxies I can scrape internally with the software meet my needs.
That said, as others have eluded to - if you will be scraping a lot of urls, or keywords you are going to need some private proxies. If you are running one simple campaign each month with limited research components, then 5 privates is probably plenty, and if you won't be scraping with it - then you probably don't really need any private proxies. You can still do considerable posting with the public proxies you can scrape with scrapebox - they're just not that good for using with Google.