Smart Pagination is the recent mode of pagination we have introduced in the RSS feed, following deprecation of page based pagination by Shopify. This ensures that all products or blog articles are included in the RSS feed in a round robin fashion, without any change in the URL.
Suppose you have set the RSS feed to display 250 items per page and your store has 1000 products, then the first feed fetch shall display products 1-250. The second, third and fourth fetch shall display products 251-500, 501-750 and 751-1000 respectively. There is no upper limit to it. Even if your store has 100,000 products, all of them shall be displayed in the RSS feed sequentially.
Earlier, there used to be a page parameter in the RSS, used for fetching specific set of products in the RSS feed. This has been deprecated due to underlying technical changes in the Shopify API.