By default, the RSS feed displays 50 items per page. You can increase this limit from 50 to any value up to 250 in the App Settings. It is not possible to increase this beyond 250
To increase this limit, open Shopify Admin and visit:
Apps > Smart RSS Feed > Product Feed > Number of Products in the Feed
However it is not recommended to increase beyond 250. You should do this only if you must have to. The number of items per page is directly proportional to the the page load time which may be increased to a level that it causes time out in the service consuming the RSS feed.
In fact, there is no need to increase the limit. Due to smart pagination, all items (products or articles) are loaded in the feed in a round-robin sequence. Suppose your store has 1000 products, then the first feed fetch shall display products 1-50, arranged in the descending order of date of creation. The second, third and fourth fetch shall display products 51-100, 101-150 and 151-200 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.