gatsby-posts.js

 // Create pages and blog posts.
 const posts = result.data.pagesGroup.edges
 const postsPerPage = 10
 const numPages = Math.ceil(posts.length / postsPerPage)

 Array.from({ length: numPages }).forEach((_, i) => {
   createPage({
     path: i === 0 ? `/blog` : `/blog/page:${i + 1}`,
     component: blogPage,
     context: {
       limit: postsPerPage,
       skip: i * postsPerPage,
       numPages,
       currentPage: i + 1,
     },
   })
 })

...