I'm not quite sure what you mean when you say "in between every posts", but I think the In-Content choices might be what you are looking for. I would also suggest using the Ezoic Chrome extension since that will make it much easier to put the placeholder exactly where you want then on the page. For a guide on how to use the Ezoic Chrome extension, you can reference this helpful support article: Using the Ezoic Chrome Extension
The Always Compete option is for if you want to ensure that the size ad is an option should the placeholder be chosen for our system.