I promised more stuff about email marketing, and here's a useful short article from Junta42 Content Marketing called What’s an Acceptable Enewsletter Open Rate? which is worth a look. The author reckons that open rates ranging from 8% to 45% are typical (it's amazing how many companies think theirs gets much higher), but mid- to high-teens is an average for decent enewsletters. So yes, that does mean that if you send out to 500 customers, don't expect more than 50-100 to actually read the thing.
An issue not mentioned in the article is that of the "open rate" statistics you're given by your emailing service or application. I would be very careful with this. It's true that many people skim through their inbox just looking at the subject lines and opening only the ones which look interesting. So that's clear: they opened it, or they didn't. But many others trudge through their inbox using the keyboard, with a preview pane open, so every email effectively gets opened, even if they don't give it a glance. Counting the real readers is almost impossible. But in the end, it's about the actions they take anyway, isn't it?