Any way to insert ad after X number of posts

There is a way but the actual implementation depends on your theme.

If your theme uses standard WP blog layout you can simply use automatic insertion
Between posts and set it to %X to insert ads every X posts:
https://adinserter.pro/documentation/automatic-insertion

Otherwise, (if your theme does not use standard WP blog layout) you can use inserton before/after HTML element:
https://adinserter.pro/documentation/automatic-insertion#before-after-html-element

You need to find out class of post elements. In order to use Filter (to insert every X posts) you would have to use Server-side insertion:
https://adinserter.pro/documentation/automatic-insertion#before-after-html-element

Please enable remode debugging and post page address so we can cehck and advise:
https://wordpress.org/support/topic/ad-inserter-support-before-you-ask-for-help/