There isn’t a fixed or ‘magic number’ when it comes to how often you should be sending emails to your list.
Sending too many emails will result in alienating and annoying your subscribers. A losing proposition.
Not sending enough emails won’t show you any kind of results, and you won’t be engaging with your list of subscribers and getting them excited about what you have to offer them. You also run the risk of them becoming bored and/or forgetting that they had signed up on your list and when you finally get around to sending an email – they unsubscribe. Another losing proposition.
A good rule of thumb for email marketing is once or twice a month. This is not an overwhelming amount of email for most people and it’s enough to keep people engaged in what you are doing.