1000, 20000 or maybe 5? There's no specific number. Simply because of many factors such as copyright skills for instance, email marketing is a big subject on it's own, you don't just build a list and make money. You need to master both traffic and conversions not just traffic.
Mastering and creating a high converting sales funnel is the key to generating sales without a big email list, focus on conversions. Come up with attractive subject lines that encourage people to open your emails, offer value to build trust then pitch once in a while and offer bonuses to encourage people buy.
It's all about how you market to your list, your email marketing skills will determine how many subscribers you need to generate a sale.