The answer is five.
Whether you hope to publish short pieces or a book, get together a list*** of five places you want to submit. Send out. As you wait for a response—could be months!—get together a second list of five. Send those out.
With ten in circulation, wait to send more until you start receiving feedback. Do, however, continue to research. This way, as soon as one rejection comes in, you have your list. Another sub goes out. Then you have your feelings.
We can allow ourselves time to grieve/resent/regret to painful side of the submission process. Unacknowledged, those feelings will eventually sink all hopes of being published and we’ll quit sending out. However, there is here is no reason to not have another sub in the pipeline before you do so.