Do each individual reviews (as a post) need separate markup?

Yes, each review (also known as a testimonial), should be marked up with it’s own schema markup using http://schema.org/Review. Then you want to figure out where you want to deploy them. If they are all on a “reviews” page, then you can do an item list, or just add the reviews to the page independently. To do this, you need to add multiple data items to one page. To do this you need to make the URI unique. For a list of reviews, you could do this by doing the URIs for each review as follows: https://www.webpage.com/reviews/#review1, https://www.webpage.com/reviews/#review2, etc. Remember to also use the field “about” for reviews, to link it to the entity being reviewed (ie. Local Business, Product, etc). Learn more about review schema markup here.