If you do an internet search for mail merge InDesign, you can find videos on YouTube that can walk you through it. What's probably happening is that you have set up a text frame for your merge fields that is too large to permit multiple instances on a single page. Maybe you used a text frame the size of the live area. If the text frame is small enough so that several can fit on a page, then it will work, unless the settings for margin and spacing are too large. You'll find those settings in the Multiple Records Layout tab of the Create Merged Document window, provided you have selected Multiple Records in the Records Per Page field.
Another common mistake is to put more than one set of placeholders on your template. Multiple records uses one set only, and does the duplication for you according to the margins and spacing you set in the options.