90% of all startups fail, and these particular founders are betting on two successful startups in a row. For the mathematically inclined out there, those founders have an approximately 1% chance of success. Good luck to them.
Why the hell would you worry about your site’s scalability if you don’t know how to effectively acquire customers? Your site falling over someday MIGHT kill your company (probably not). Never growing past eight customers WILL kill your company. Instead of using your beautiful brain and precious funding to solve theoretical scaling issues, perhaps you should go run some customer acquisition experiments. Build a fucking landing page. Buy some AdWords.