How many emails to send before asking for the sale?

There is no widely held opinion or statistic showing the exact number of emails that should go out before asking for the sale. 

If the only time your email list hears from you is when you're pitching an offer, it becomes clear that you don't care about them, you only care about the sale. They're going to unsubscribe or ignore your emails.

Instead of providing valuable content, you're asking for their wallet constantly.

Think about the last time you were interested in a particular subject so you poked around online trying to learn more about it. Once you found a valuable website, you probably read everything they had to offer. Lastly, you contemplated purchasing their paid, more intensive version of knowledge or process.

You didn't start off wanting to purchase something. You were led into it by their valuable content. It became clear by watching their videos, listening to their podcast, or reading their blog that you connected with them and wanting to learn more.

When thinking of an email sequence leading to a sale, consider the know, like and trust factor that is more important than the sale itself. Providing valuable free content is a part of that.