Wordpress Pagination

I have a wordpress page that I want to Paginate posts on but I only want to show 18 posts at a time and then on scroll down, load the next 18. so if you have a total of 20 items at first it should show 18. When you scroll down, it kicks in and shows the last 2
that's all there is to it
now if you have 60 items, then it will kick in after the 18th..... it will load another 18
once your reached the bottom it will reach the other 18 and so on.  Can anyone point my in the correct direction?
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Peter HartCommented:
what you are describing sounds like the "infinite scroll" or "load new posts in scroll" function.
I don't think you can state a number like 18, but it just loads the next posts that fit in the bit you have scrolled.
there is a way with JetPack  discussed here:
but states it only works on infinite scroll compatible themes.
there is also this plugin

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