You can create In-Feed ads (ads that will be shown in between posts) on the blog page before a particular numbered post. In order to create In-Feed ads, go to Settings → Placements → Add Rule. Here, under the Placement type, select In Feed option and also provide post number before which you want to display the ad. You can align ad to left, right, center or keep alignment to none. You can see the URL of the page your in-feed ads will be displayed on.